Live Merge: Suspend disk stats collection during pivot

When pivoting to the new volume after an active layer merge, there is a
brief period where the vdsm metadata is out of sync.  During this time,
the disk stats collection thread will raise an exception since the drive
cannot be referred to by its old path.  Therefore we disable stats
collection during this short window until the metadata can be
synchronized.

Change-Id: I08689aec4d61871a568a6f92661b560fbf4d0b57
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1109918
Signed-off-by: Adam Litke <alitke@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/31367
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>