Improve usability of manipulating various VMs disk sizes

[bnvk]

Earlier today I encountered the following error message upon booting up a StandaloneVM

It was perplexing as I was only using 21GB of the 26GB allotted, but I figured it was something like I encountered with /tmp directory in issue #1191

After searching the mailing list and the docs, I first thought "Resizing Disk Image" was the right thing. I tried a couple of the steps in there to no avail. Then I tried "
Resizing root.img Size" steps and issue fixed. All told this took me about 30 minutes. So, couple ideas:

• Make the wording in the notification and the documentation better match (i'm doing this right now)
• Make it relatively intuitive to explain & do these things from the VM's GUI settings

[marmarek]

Actually this notification is nothing Qubes specific. It is just standard (gnome?) application installed by default in template.

[marmarek]

We have both "private" and "system" (aka "root") volume sizes configurable in GUI qube settings. The "system" one is active only for qubes having own system volume (templates, standalone).
I see two possible improvements here:

• for template-based qubes, add some info that "system" volume size can be changed in template's settings
• add info about current usage

The second one may be slightly misleading, as it would be from the time the settings are open, not updated in real-time. So, if qube is writing a lot of data, the number will be outdated. But IMO it's very minor issue.

@marmarta do we want this, or leave it as it is?

[marmarta]

We have both "private" and "system" (aka "root") volume sizes configurable in GUI qube settings. The "system" one is active only for qubes having own system volume (templates, standalone).
I see two possible improvements here:

* for template-based qubes, add some info that "system" volume size can be changed in template's settings

This is done by QubesOS/qubes-manager#141. Also, I made a pull request to improve the documentation to make it less intimidating ( QubesOS/qubes-doc#765 ) because honestly, when I looked at it myself I really did not think "oh nice just use the GUI to resize" but more like "so many steps, so much can go wrong")

* add info about current usage

Maybe this should rather be added to the Qube Manager? We have current usage but not maximum size; maybe it would be a good idea to add something like % used.