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: Lucas Meneghel Rodrigues <lmr@redhat.com>
qemu: unittests - Handle running on qemu upstream new testdevs

The implementation of the devices needed for KVM unit
testing changed a bit on qemu:

1) There's no need for a testlog anymore (as kvm-unit-tests
   now writes to the standard serial port)
2) Now there's a shared new device, called isa-debug-exit,
   that when written to, causes qemu to exit
3) Due to the new behavior of the isa-debug-exit device,
   the new known good behavior for qemu at the end of the
   unit test is to return 1.

Take that all into account, while still keeping backwards
compatibility. Also, document how one can run the unittests
with the new virt-tests runner both in the config file
and the wiki.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>