Add header files with structs and defines for the virtio serial device.
The virtio serial device also known as virtio console device because
initially it had only support for a single tty, intended to be used as
console. Support for multiple streams and named data ports has been
added later on.
https://docs.oasis-open.org/virtio/virtio/v1.2/cs01/virtio-v1.2-cs01.html#x1-2900003
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Gerd Hoffmann