Live Merge: Extend internal block volumes during merge

Libvirt will be exposing the high write watermark for internal volumes
directly in the domain XML.  An <allocation/> element will be added to
each member of the image chain.  This patch implements the missing logic
for _getMergeWriteWatermarks using this new API.

Change-Id: I3a9e0ebdb9c42df713c40e0fc5782945eb7228a8
Signed-off-by: Adam Litke <alitke@redhat.com>
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1109920
Reviewed-on: http://gerrit.ovirt.org/31268
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>