r/virtual_machine: Correct present NIC device calculation

[vancluever]

This corrects an issue with the way NIC devices were checked to see if
the slots they are supposed to occupy on the PCI bus were already used
or not. An offset that was used to populate the current device range was
not being used when the devices were actually checked, creating invalid
results and out of range errors.

Fixes #279.

[vancluever]

Test passes:

=== RUN   TestAccResourceVSphereVirtualMachine
=== RUN   TestAccResourceVSphereVirtualMachine/maximum_number_of_nics
--- PASS: TestAccResourceVSphereVirtualMachine (187.24s)
    --- PASS: TestAccResourceVSphereVirtualMachine/maximum_number_of_nics (187.24s)

[mbfrahry]

Just a minor thing

[mbfrahry]

Do we need to add pciDeviceOffset if we are just subtracting it 2 lines later? Line 872 for reference

[vancluever]

Yeah, units is a []bool of length 10 that we use to get a "map" of what PCI devices in the available network device PCI range (7-16) are currently taken to make sure we are not going to add/assign a device in an already occupied slot. This obviously has an index range of 0-9, and is not reflective of the final unit number. This was actually an error as the slice is being correctly populated here, but not being subsequently referenced correctly.

This is why only 3 devices can be added right now as per the referenced issue:

network_interface.0 7
network_interface.1 8
network_interface.2 9
network_interface.3 10 (out of range here)

[mbfrahry]

Ah. Yep! Makes sense now. Thank you

[mbfrahry]

LGTM!

[vancluever]

Thanks @mbfrahry!