volumeTests: add BlockDomainMetadataSlotTest

A new test has been added to check the metadata slot selection.

Change-Id: I0ff018625443ce7cf75d3edf11644544e7f23dde
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26266
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>