Map the new <panic> device in XML to the '-device pvpanic' command
line of qemu.  Clients can then couple the <panic> device and the
<on_crash> directive to control behavior when the guest reports
a panic to qemu.
Signed-off-by: NHu Tao <hutao@cn.fujitsu.com>
Signed-off-by: NEric Blake <eblake@redhat.com>