postcopy: Check for shared memory

Postcopy doesn't support migration of RAM shared with another process
yet (we've got a bunch of things to understand).
Check for the case and don't allow postcopy to be enabled.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
