vhost-vdpa: refine ioctl pre-processing

Switch to use 'switch' to make the codes more easier to be extended.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20200804162048.22587-2-eli@mellanox.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>