Postcopy: Fix TP!=HP zero case

Where the target page size is different from the host page
we special case it, but I messed up on the zero case check.

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>
