xen/9pfs: connect to the backend

xen/9pfs: connect to the backend

Implement functions to handle the xenbus handshake. Upon connection,
allocate the rings according to the protocol specification.

Initialize a work_struct and a wait_queue. The work_struct will be used
to schedule work upon receiving an event channel notification from the
backend. The wait_queue will be used to wait when the ring is full and
we need to send a new request.

CC: [email protected]
CC: [email protected]
CC: [email protected]
CC: Eric Van Hensbergen <[email protected]>
CC: Ron Minnich <[email protected]>
CC: Latchesar Ionkov <[email protected]>
CC: [email protected]
Signed-off-by: Stefano Stabellini <[email protected]>
Reviewed-by: Juergen Gross <[email protected]>
Signed-off-by: Juergen Gross <[email protected]>