introduce a new qom device to deal with panicked event

pvpanic device is used to send guest panic event from guest to qemu.

When guest panic happens, pvpanic device driver will write a event
number to IO port 0x505(which is the IO port occupied by pvpanic device,
by default). On receiving the event, pvpanic device will pause guest
cpu(s), and send a qmp event QEVENT_GUEST_PANICKED.

Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
Signed-off-by: Hu Tao <hutao@cn.fujitsu.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-id: b66077a40235b3531632a05a6ff373850afc7d2e.1366945969.git.hutao@cn.fujitsu.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
