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>
diff --git a/qemu-options.hx b/qemu-options.hx
index eb55cd0..94cdfcf 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -3899,6 +3899,8 @@
     "                   action when guest reboots [default=none]\n"
     "-action shutdown=poweroff|pause\n"
     "                   action when guest shuts down [default=poweroff]\n"
+    "-action panic=poweroff|pause|none\n"
+    "                   action when guest panics [default=poweroff]\n"
     "-action watchdog=reset|shutdown|poweroff|inject-nmi|pause|debug|none\n"
     "                   action when watchdog fires [default=reset]\n",
     QEMU_ARCH_ALL)
@@ -3911,6 +3913,7 @@
 
     Examples:
 
+    ``-action panic=none``
     ``-action reboot=shutdown,shutdown=pause``
     ``-watchdog i6300esb -action watchdog=pause``