xen/pvcallsback: use lateeoi irq binding

In order to reduce the chance for the system becoming unresponsive due
to event storms triggered by a misbehaving pvcallsfront use the lateeoi
irq binding for pvcallsback and unmask the event channel only after
handling all write requests, which are the ones coming in via an irq.

This requires modifying the logic a little bit to not require an event
for each write request, but to keep the ioworker running until no
further data is found on the ring page to be processed.

This is part of XSA-332.

Cc: stable@vger.kernel.org
Reported-by: Julien Grall <julien@xen.org>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Wei Liu <wl@xen.org>