vfio: fix adding memory listener to the right address space

Depending on the device, container->space->as contains the valid AddressSpace.
Using address_space_memory breaks devices sitting behind an iommu (and using
a separate address space).

Signed-off-by: Frank Blaschka <blaschka@linux.vnet.ibm.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
