address_space_destroy_dispatch is called from an RCU callback and hence
outside the iothread mutex (BQL).  However, after address_space_destroy
no new accesses can hit the destroyed AddressSpace so it is not necessary
to observe changes to the memory map.  Move the memory_listener_unregister
call earlier, to make it thread-safe again.

Reported-by: Alex Williamson <alex.williamson@redhat.com>
Fixes: 374f2981
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>