virtio: make VIRTIO a menuconfig to ease disabling it all

No need to get into the submenu to disable all VIRTIO-related
config entries.

This makes it easier to disable all VIRTIO config options
without entering the submenu. It will also enable one
to see that en/dis-abled state from the outside menu.

This is only intended to change menuconfig UI, not change
the config dependencies.

Signed-off-by: Vincent Legoll <vincent.legoll@gmail.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>