vl: Add option to avoid stopping VM upon guest panic

The current default action of pausing a guest after a panic event
is received leaves the responsibility to resume guest execution to the
management layer. The reasons for this behavior are discussed here:
https://lore.kernel.org/qemu-devel/52148F88.5000509@redhat.com/

However, in instances like the case of older guests (Linux and
Windows) using a pvpanic device but missing support for the
PVPANIC_CRASHLOADED event, and Windows guests using the hv-crash
enlightenment, it is desirable to allow the guests to continue
running after sending a PVPANIC_PANICKED event. This allows such
guests to proceed to capture a crash dump and automatically reboot
without intervention of a management layer.

Add an option to avoid stopping a VM after a panic event is received,
by passing:

-action panic=none

in the command line arguments, or during runtime by using an upcoming
QMP command.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Alejandro Jimenez <alejandro.j.jimenez@oracle.com>
Message-Id: <1607705564-26264-3-git-send-email-alejandro.j.jimenez@oracle.com>
[Do not fix panic action in the variable, instead modify -no-shutdown. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

qapi/run-state.json

@@ -353,6 +353,18 @@
 { 'enum': 'ShutdownAction',
   'data': [ 'poweroff', 'pause' ] }
 
+##
+# @PanicAction:
+#
+# @none: Continue VM execution
+#
+# @pause: Pause the VM
+#
+# Since: 6.0
+##
+{ 'enum': 'PanicAction',
+  'data': [ 'poweroff', 'pause', 'none' ] }
+
 ##
 # @watchdog-set-action:
 #
@@ -372,6 +384,8 @@
 #
 # @shutdown: @ShutdownAction action taken on guest shutdown.
 #
+# @panic: @PanicAction action taken on guest panic.
+#
 # @watchdog: @WatchdogAction action taken when watchdog timer expires .
 #
 # Returns: Nothing on success.
@@ -383,12 +397,14 @@
 # -> { "execute": "set-action",
 #      "arguments": { "reboot": "shutdown",
 #                     "shutdown" : "pause",
+#                     "panic": "pause",
 #                     "watchdog": "inject-nmi" } }
 # <- { "return": {} }
 ##
 { 'command': 'set-action',
   'data': { '*reboot': 'RebootAction',
             '*shutdown': 'ShutdownAction',
+            '*panic': 'PanicAction',
             '*watchdog': 'WatchdogAction' },
   'allow-preconfig': true }