qemu: Refactor default panic model

qemu: Refactor default panic model

Perform decisions based on the architecture and machine type
in a single place instead of duplicating them.

This technically adds new behavior for MODEL_ISA in
qemuDomainDefAddDefaultDevices(), but it doesn't make any
difference functionally since we don't set addPanicDevice
outside of ppc64(le) and s390(x). If we did, the lack of
handling for that value would be a latent bug.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>