Size values in the blueprint can be defined as either integers,
representing bytes, or strings with SI unit prefixes for bytes.
The conversion for the Filesystem.MinSize happens in both the JSON and
TOML unmarshallers.  With the new partitioning customizations we will
have more conversions of the same type.

This commit extracts the handling into a generic function for
reusability and convenience.

This changes the error messages.  Tests have been updated accordingly.

- New structure that's more powerful than the existing array of
  FilesystemCustomization.
- The general structure defines a (minimum) size for the overall disk
  and an array of partitions.
- Each partition has one of three types:
  - plain: a plain disk partition, meaning partitions with traditional
    filesystem payloads (xfs, ext4, etc).
  - lvm: a partition containing an LVM volume group, which in turn
    will contain one or more LVM logical volumes.
  - btrfs: a partition containing a btrfs volume, which in turn will
    contain one or more subvolumes.
- The new FilesystemTypedCustomization struct supports a label and
  fs_type and no minsize, compared to the old FilesystemCustomization.
  This struct is used to define both the payload of a plain partition
  and the payload of an LVM logical volume.
- The PartitionCustomization has a type and size and embeds three
  substructures, one for each partition type.  Decoding/unmarshalling of
  the PartitionCustomization is handled by a "typeSniffer" which first
  reads the value of the "type" field and then deserialises the whole
  object into a struct that only contains the fields valid for that
  partition type.  This ensures that no fields are set for the
  substructure of a different type than the one defined in the "type"
  fields.

The unmarshaller decodes "type" and "minsize" manually and reuses the
decode* functions from the JSON unmarshaller for each subtype based on
the value of "type".

Add validation functions for DiskCustomization.  Three types of
validation are added:
- Validate(): checks for functionally invalid configurations (bad
  mountpoint names or invalid structures, etc).  The docstring lists all
  properties that are validated.
- ValidateLayoutConstraints(): checks for limitations we imposed
  ourselves as a matter of policy and support on the structure of the
  partition table.  This includes disallowing, for example, multiple LVM
  volume groups, multiple btrfs volumes, or combinations of the two.
  While these layouts are technically valid for a partitioned disk, we
  currently do not support them.
- CheckDiskMountpointsPolicy(): Checks that none of the mountpoints are
  disallowed by the given PathPolicy.

Tests added for all validators.

Define a nested map:
[partition table type] -> [partition type name] -> [partition type ID]
and use it from a private function getPartitionTypeIDfor() for
convenient partition type ID selection.

New function that creates a partition table from scratch based on the
new disk customization.

First, it creates any necessary partitions for the selected boot mode
(BIOS boot, ESP, or both).
Then it determines if a /boot partition is needed based on the
partitions and mountpoints defined in the disk customizations.
Then, it iterates through partitions and creates each one in the order
its defined.
Finally, it creates a root filesystem if one wasn't already defined,
resizes any necessary volumes, relayouts the table (setting all
partition starts and offsets), and generates UUIDs all entities.

Because of the new way the boot partition is handled, the
EnsureBootPartition() function was changed to always add a boot
partition and was renamed to AddBootPartition() to match the new
behaviour.  The tests have been updated accordingly.

Preparing the function to use the new PartitioningCustomization when
set.

For all image types that don't specify a requiredPartitionSizes, add the
defaults that are used in the NewPartitionTable() so that
NewCustomPartitionTable() has similar behaviour.

The only image type that explicitly doesn't specify minimum sizes is the
iot-raw-image, which has a pre-existing empty map.

Add checks for Partitioning customizations in Fedora's checkOptions()
function.  The function now returns an error if:
- Partitioning is defined at the same time as Filesystem
- Partitioning is defined for ostree image types (commit and container).
- The mountpoints in Partitioning violate the mountpoint policies for
  the image type.

One for each variant:
- Plain
- Plain + btrfs
- Plain + lvm