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: NLucas Meneghel Rodrigues <lmr@redhat.com>