Added support for generating an XFS UsrOnly root

[mcassaniti]

When the options UsrOnly = yes and Format = gpt_xfs are combined, mkosi throws an exception because the FS image is never created or populated. There is no error beforehand that this combination of options is invalid.

The only issue I can see with the code I've added is that XFS doesn't seem to support a minimize option similar to ext4 after the FS is populated.

[mcassaniti]

Anything else holding this one up?

[DaanDeMeyer]

I wrote these comments ages ago but forgot to submit, apologies

[mcassaniti]

Thanks for all the effort @DaanDeMeyer