volumeTests: avoid AttributeError during __del__

Since sd.StorageDomain.__del__() uses its self.stat,
FileDomainMockObject must expose it.

Change-Id: Ibfc602f419a7e1c95b9a86c58a7dbb561de8a625
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/23929
Reviewed-by: Ondřej Svoboda <osvoboda@redhat.com>
Tested-by: Ondřej Svoboda <osvoboda@redhat.com>