watchdog: Allow setting action on the fly

Currently, the only time that users can set watchdog action is at
the start as all we expose is this -watchdog-action command line
argument. This is suboptimal when users want to plug the device
later via monitor. Alternatively, they might want to change the
action for already existing device on the fly.

Inspired by: https://bugzilla.redhat.com/show_bug.cgi?id=1447169

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Message-Id: <35d6ce6fe3d357122d73b8272bc8198134c74104.1504771369.git.mprivozn@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
[Missing colon in doc comment fixed]
Signed-off-by: Markus Armbruster <armbru@redhat.com>
