xenbus: notify the other end when necessary

xenbus: notify the other end when necessary

The xenbus thread didn't send notification to other end when it expected
more data or consumed responses, which led to stalling the ring from
time to time.

This is the culprit that guest was less responsive when using stubdom
because the device model was stalled.

Fix this by sending notification to the other end when it consumes a
message. A bunch of memory barriers are also added to ensure
correctness.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Samuel Thibault <samuel.thibault@ens-lyon.org>