Mini-OS: add 9pfs transport layer

Mini-OS: add 9pfs transport layer

Add the transport layer of 9pfs. This is basically the infrastructure
to send requests to the backend and to receive the related answers via
the rings.

As a first example add the version and attach requests of the 9pfs
protocol when mounting a new 9pfs device. For the version use the
"9P2000.u" variant, as it is the smallest subset supported by the qemu
based backend.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>