machined: if /var/lib/machines is backed by a loop file, resize it on "machinectl...

machined: if /var/lib/machines is backed by a loop file, resize it on "machinectl set-limit"

When the pool size limit is altered with "machinectl set-limit", then
not only set the subvolume quota of the /var/lib/machine subvolume, but
also resize the backing loop file and the btrfs file system on it
dynamically.