run: Add kvm specific option --accel

Make it possible to choose between tcg and kvm when running
the kvm specific tests. This is part of a series of changes
that will end up in renaming what we know today as 'kvm'
test to 'qemu'.

I think there's a bunch of assumptions that we make throughout
the tests with regards to qemu being accelerated with kvm,
that we'll discover when running the tests with tcg. Stay
tuned.
Signed-off-by: NLucas Meneghel Rodrigues <lmr@redhat.com>