[3/5] xen/xenbus/xen_bus_type: Support will_handle watch callback

[3/5] xen/xenbus/xen_bus_type: Support will_handle watch callback

Origin: https://git.kernel.org/linus/be987200fbaceaef340872841d4f7af2c5ee8dc3
Bug: https://xenbits.xen.org/xsa/advisory-349.html
Bug-Debian-Security: https://security-tracker.debian.org/tracker/CVE-2020-29568

This commit adds support of the 'will_handle' watch callback for
'xen_bus_type' users.

This is part of XSA-349

Cc: stable@vger.kernel.org
Signed-off-by: SeongJae Park <sjpark@amazon.de>
Reported-by: Michael Kurth <mku@amazon.de>
Reported-by: Pawel Wieczorkiewicz <wipawel@amazon.de>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Gbp-Pq: Topic bugfix/all
Gbp-Pq: Name xen-xenbus-xen_bus_type-Support-will_handle-watch-ca.patch