sp: move validatePoolSD in the StoragePool class

It's the StoragePool duty to validate if an operation cannot be
accomplished because the storage domain is not part of the pool.

For consistency with the other pool Vms API the sdUUID argument
in the getVmsList method is now mandatory (it is set by the HSM
public method).

Change-Id: I75f0245ec5449d57431df7055be401d73975312b
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21787
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Ayal Baron <abaron@redhat.com>