run: Give more helpful message when user chooses an invalid test type

This is a solution for issue #175.

With the recent switch kvm-> qemu, it's natural that
people mistake those for a while. We haven't been handling
that case well. So let's stablish valid test types and,
if the user chooses an invalid one, give him/her an
understandable error message.

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