Acceptance tests: add make rule for running them

The acceptance (aka functional, aka Avocado-based) tests are
Python files located in "tests/acceptance" that need to be run
with the Avocado libs and test runner.

Let's provide a convenient way for QEMU developers to run them,
by making use of the tests-venv with the required setup.

Also, while the Avocado test runner will take care of creating a
location to save test results to, it was understood that it's better
if the results are kept within the build tree.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Acked-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Reviewed-by: Caio Carrara <ccarrara@redhat.com>
Message-Id: <20181018153134.8493-3-crosa@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>