qmp: generalize watchdog-set-action to -no-reboot/-no-shutdown

qmp: generalize watchdog-set-action to -no-reboot/-no-shutdown

Add a QMP command to allow for the behaviors specified by the
-no-reboot and -no-shutdown command line option to be set at runtime.
The new command is named set-action and takes optional arguments, named
after an event, that provide a corresponding action to take.

Example:

-> { "execute": "set-action",
     "arguments": {
	"reboot": "none",
	"shutdown": "poweroff",
	"watchdog": "debug" } }
<- { "return": {} }

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Alejandro Jimenez <alejandro.j.jimenez@oracle.com>
Message-Id: <1607705564-26264-4-git-send-email-alejandro.j.jimenez@oracle.com>
[Split the series differently, with -action based on the QMP command. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>