Live Merge: Fix backing chain xml parsing

The backingStore element is optional inside of a disk XML block.  It
will be missing for disks with no backing files (ie. new VM based on
the blank template).  Handle this case by breaking out of the loop
rather than returning an error.

Change-Id: I6128d1eb1265f7ede1ab420ac7b5342bc7f2db9e
Signed-off-by: Adam Litke <alitke@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/29006
Reviewed-by: Dan Kenigsberg <danken@redhat.com>