xen-hvm: Allow disabling buffer_io_timer

xen-hvm: Allow disabling buffer_io_timer

commit f37f29d31488 "xen: slightly simplify bufioreq handling" hard
coded setting req.count = 1 during initial field setup before the main
loop.  This missed a subtlety that an early exit from the loop when
there are no ioreqs to process, would have req.count == 0 for the return
value.  handle_buffered_io() would then remove state->buffered_io_timer.
Instead handle_buffered_iopage() is basically always returning true and
handle_buffered_io() always re-setting the timer.

Restore the disabling of the timer by introducing a new handled_ioreq
boolean and use as the return value.  The named variable will more
clearly show the intent of the code.

Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Reviewed-by: Paul Durrant <paul@xen.org>
Message-Id: <20211210193434.75566-1-jandryuk@gmail.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
(cherry picked from commit 9288e803e61e8d56d1c6c6aa8beb58596fb84ed9)
[perard: fix context]