tools: xen-ringwatch: spot, and work around, broken frontend/backend ring I/O

tools: xen-ringwatch: spot, and work around, broken frontend/backend ring I/O

Adds tool support to debug backends which expose I/O ring state in
sysfs. Currently supports /sys/devices/xen-backend/vbd-*-*/io_ring
nodes for block I/O, where implemented.

Primary function is to observe ring state make progress over a period
of time, then report stuck message queue halves where pending
consumer/event are not moving.

Adding --kick will re-issue event notifications to frontends, and/or
kick backends out of wait state.

Signed-off-by: Daniel Stodden <daniel.stodden@citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>