qemu: Forbid ib700 watchdogs for non-i440fx machine types

qemu: Forbid ib700 watchdogs for non-i440fx machine types

We can launch qemu with it, but it will not work since it's not even
probed by the kernel at the mapped address with different machine types
since they are expected to be connected to ISA and not even its newer
LPC counterpart found on q35.  And it does not exist on non-x86
architectures.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>