xen/9pfs: send requests to the backend

Implement struct p9_trans_module create and close functions by looking
at the available Xen 9pfs frontend-backend connections. We don't expect
many frontend-backend connections, thus walking a list is OK.

Send requests to the backend by copying each request to one of the
available rings (each frontend-backend connection comes with multiple
rings). Handle the ring and notifications following the 9pfs
specification. If there are not enough free bytes on the ring for the
request, wait on the wait_queue: the backend will send a notification
after consuming more requests.

CC: groug@kaod.org
CC: jgross@suse.com
CC: Eric Van Hensbergen <ericvh@gmail.com>
CC: Ron Minnich <rminnich@sandia.gov>
CC: Latchesar Ionkov <lucho@ionkov.net>
CC: v9fs-developer@lists.sourceforge.net

Signed-off-by: Stefano Stabellini <stefano@aporeto.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>