pseries: Correct panic behaviour for pseries machine type

The pseries machine type doesn't usually use the 'pvpanic' device as such,
because it has a firmware/hypervisor facility with roughly the same
purpose.  The 'ibm,os-term' RTAS call notifies the hypervisor that the
guest has crashed.

Our implementation of this call was sending a GUEST_PANICKED qmp event;
however, it was not doing the other usual panic actions, making its
behaviour different from pvpanic for no good reason.

To correct this, we should call qemu_system_guest_panicked() rather than
directly sending the panic event.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Thomas Huth <thuth@redhat.com>
