qemu: add panic device support for S390

qemu: add panic device support for S390

If a panic device is being defined without a model in a domain
the default value is always overwritten with model ISA. An ISA
bus does not exist on S390 and therefore specifying a panic device
results in an unsupported configuration.
Since the S390 architecture inherently provides a crash detection
capability the panic device should be defined in the domain xml.

This patch adds an s390 panic device model and prevents setting a
device address on it.

Signed-off-by: Boris Fiuczynski <fiuczy@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>