libvirt-machine-props: Refresh volume info after resize

For running machines, we make use of special libvirt API that operates
on GVir.DomainDisk rather than GVir.StorageVol and its the latter Boxes
reads current capacity info from (actually there is no way to read
capacity from GVir.DomainDisk) so what happens is that when you change the
capacity of a running machine, get out of properties and back again,
you find the capacity to remain unchanged even though the actual
capacity was changed.

This patch fixes the issue by refreshing the storage pool and refetching
the volume info after changing the capacity through GVir.DomainDisk.resize.