qemu_vm: Update testdev option, add -isa-debug-exit option

For qemu upstream, in order to run the kvm-unit-tests suite,
there are new test devices, that differ in syntax from the
ones present in qemu-kvm:

* pc-testdev
* isa-debug-exit

Add them both to the qemu VM class, and make things
backwards compatible (ie, if an older qemu-kvm is being
tested, it'll handle the correct syntaxes as well).
Signed-off-by: NLucas Meneghel Rodrigues <lmr@redhat.com>