xen: cleanup IOREQ server on exit

xen: cleanup IOREQ server on exit

Use the backported Notifier interface to register an atexit handler to
cleanup the IOREQ server. This is required since Xen commit a5a180f9
("x86/domain: don't destroy IOREQ servers on soft reset") is introduced
which requires Qemu to explicitly close the IOREQ server.

This is can be seen as a backport of ba7fdd64 ("xen: cleanup IOREQ
server on exit").

Signed-off-by: Maximilian Heyne <mheyne@amazon.de>
Reviewed-by: Paul Durrant <paul@xen.org>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>