sd: unify validateCreateVolumeParams

One validateCreateVolumeParams is enough, there's no real need to have
it both in the domain and volume object.

Change-Id: I082dc8b382af4a163fd4d10c67e696b1072e6b71
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28128
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>