vfio: Eliminate vfio_container_ioctl()

vfio_container_ioctl() was a bad interface that bypassed abstraction
boundaries, had semantics that sat uneasily with its name, and was unsafe
in many realistic circumstances.  Now that spapr-pci-vfio-host-bridge has
been folded into spapr-pci-host-bridge, there are no more users, so remove
it.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Acked-by: Alex Williamson <alex.williamson@redhat.com>
