sp: fix pool membership check in attachSD

A race between attachStorageDomain and connectStoragePool using
StoragePoolMemoryBackend could lead to the new domain not being
attached (on storage).

The race happens in StoragePool.attachSD:

        domains = self.getDomains()
        if sdUUID in domains:
            return True

if the domain map has been already updated by connectStoragePool
the domain is deemed attached and the remaining operations are
skipped.

In this patch:
- use only validateAttachedDomain in detachSD (validatePoolSD is
  redundant)
- make attachSD symmetric to detachSD using validateAttachedDomain
  to check if the domain is already attached

Change-Id: I4169ddcb4da29c8b806b5788080c16349b642197
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/23446
Reviewed-by: Dan Kenigsberg <danken@redhat.com>