domain: remove race condition in extend

During a domain extension the metadata mappings are checked against
MAX_PVS, in doing so the self._extendlock should be acquired to prevent
multiple concurrent extensions that would surpass the MAX_PVS number.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=876663
Change-Id: Iec2a0762dc2b002bcf787653bc4508a760f71c60
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/11278
Reviewed-by: Dan Kenigsberg <danken@redhat.com>