virtio_pci_modern: switch to type-safe io accessors

As Rusty noted, we were accessing queue_enable with an incorrect width.
Switch to type-safe accessors so we don't make this mistake again in the
future.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>