Draft 21

virtio-msg: propose common-spec integration text

Update common-spec text so Virtio Over Messages is referenced
from the core device facilities and introduction.

This is a proposal for changes in the virtio common
specification to properly insert the virtio message transport
in the transport-neutral text.

Keep the change minimal for now:
- update content.tex to mention Virtio Over Messages in the
  transport list and in VIRTIO_F_ADMIN_VQ text
- update introduction.tex to mention message-based transports

We could also adapt newtransport.tex, but that is intentionally
left out here to keep the proposal minimal.

Signed-off-by: Bertrand Marquis <bertrand.marquis@arm.com>