qemu: use a wrong name for guest panic status

qemu: use a wrong name for guest panic status

https://bugzilla.redhat.com/show_bug.cgi?id=1178652

We will get a warning when we have a guest in paused
status (caused by kernel panic) and restart libvirtd,
warning message like this:

Qemu reported unknown VM status: 'guest-panicked'

and this seems because we set a wrong status name in
qemu_monitor.c, and from qemu qapi-schema.json file
we know this status should named 'guest-panicked'.

Signed-off-by: Luyao Huang <lhuang@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>