vm: update volume apparentsize after snapshot

After a live snapshot succeeded we need to force the update of the
volume apparentsize to avoid races in the extension requests.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=923964
Change-Id: I472c2931551643914e6a09b54d3d96f371f34864
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/13346
Reviewed-by: Vered Volansky <vvolansk@redhat.com>
Reviewed-by: Ayal Baron <abaron@redhat.com>