sp: ensure that master domain is active

It's not possible to be connected to a pool where the master domain
is attached (and not active).
In case the engine tries to send a domain map (memory backend) in
which the master domain is attached the operation should fail.

Change-Id: I4e8fe07ebf8e59e9b028678fecd0a24578348b6c
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28331
Reviewed-by: Dan Kenigsberg <danken@redhat.com>