libvirt-machine-props: Volume size is correct units

GVir.DomainDisk.resize expects values in KiB.

I'm not entirely sure if it helps with the following bug but my guess is
that it does as the disk capacity really must have been set to that
insane value because of the issue this bug fixes.

https://bugzilla.gnome.org/show_bug.cgi?id=725384