Don't bump the receiver event counter if the poller is going to retry

Don't bump the receiver event counter if the poller is going to retry

There is little point in bumping rsp_event (to trigger a new event from
the backend) if the poller is going to retry, so we can save modifying the
shared ring in this case.

This patch also adds extra debug code to the poller to make sure it never
exits from the main loop until either there are no retries pending or
the instance has been disabled.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>