sp: specific method to validate the master version

The validation of the master domain version must be abstracted as
it relies on directly accessing the pool metadata.

In this patch:
- remove (the barely unused) validatePoolMVerHigher
- add a specific method in charge to validate the master version on
  a specific domain (validateMasterDomainVersion)

Change-Id: I8a055c9eb21cc5681d1f9afde5e7eab899aa65c0
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/22130
Reviewed-by: Ayal Baron <abaron@redhat.com>