core: fix race when removing memory volumes during Live Merge

Memory volume removal was not tracked as a child command, and its
completion would trigger endAction of RemoveSnapshotCommand before the
tasks run by ExecutionHandler were complete.  Instead, use the newer
RemoveMemoryVolumesCommand during Live Merge, which Executionhandler can
coordinate.

Change-Id: I12e317ada7159101cc5349e9a084b7f316493048
Bug-Url: https://bugzilla.redhat.com/1122691
Related-To: https://bugzilla.redhat.com/1083310
Signed-off-by: Greg Padgett <gpadgett@redhat.com>