s390x/watchdog: introduce diag288 watchdog device

s390x/watchdog: introduce diag288 watchdog device

This patch introduces a new diag288 watchdog device that will, just like
other watchdogs, monitor a guest and take corresponding actions when it
detects that the guest is not responding.

diag288 is s390x specific. The wiring to s390x KVM will be done in
separate patches.

Signed-off-by: Xu Wang <[email protected]>
Reviewed-by: David Hildenbrand <[email protected]>
Signed-off-by: Christian Borntraeger <[email protected]>
[split out qemu-option.hx base changes]