The recent introduction of a bus master container added
memory_region_add_subregion() into the PCI device registering path but
missed memory_region_del_subregion() in the unregistering path leaving
a reference to the root memory region of the new container.
This adds missing memory_region_del_subregion().
Fixes: 3716d5902d743 ("pci: introduce a bus master container") Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru> Reviewed-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>