virtio: Helper for registering virtio device types

virtio: Helper for registering virtio device types

Introduce a helper for registering different flavours of virtio
devices.  Convert code to use the helper, but keep only the
existing generic types.  Transitional and non-transitional device
types will be added by another patch.

Acked-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>