xen-block: improve response latency

xen-block: improve response latency

If the I/O ring is full, the guest cannot send any more requests
until some responses are sent. Only sending all available responses
just before checking for new work does not leave much time for the
guest to supply new work, so this will cause stalls if the ring gets
full. Also, not completing reads as soon as possible adds latency
to the guest.

To alleviate that, complete IO requests as soon as they come back.
xen_block_send_response() already returns a value indicating whether
a notify should be sent, which is all the batching we need.

Signed-off-by: Tim Smith <tim.smith@citrix.com>
Re-based and commit comment adjusted.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>