qemu: Sometimes the default panic model doesn't exist

qemu: Sometimes the default panic model doesn't exist

Right now the fallback behavior is to use MODEL_ISA if we
haven't been able to find a better match, but that's not very
useful as we're still going to hit an error later, when
QEMU_CAPS_DEVICE_PANIC is not found at Validate time.

Instead of doing that, allow MODEL_DEFAULT to get all the
way to Validate and report an error upon encountering it.

The reported error changes slightly, but other than that the
set of configurations that are allowed and blocked remains
the same.

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