qemu: Simplify test APIs

After feedback received during the sprint review, the
idea is to greatly simplify how virt-tests behave and
work. This patch basically:

* Ditches manual setup by moving the setup stages to
the test.VirtTest class, inside the avocado-virt plugin.
* Changes the API calls from .launch() and shutdown() to
power_on() and power_off(), as decided during sprint
meeting.

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