virt: change test entry point method name

There's no need to have the name of the "run_<name>" method (main
test entry point) depend on the test name. So, for simplicity sake,
let's just name the test entry point "run".

I have decided to not fallback gracefully to the previous naming
convention as I believe this can lead to confusion in the future.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
virt: change missing references to run method name

Signed-off-by: Cleber Rosa <crosa@redhat.com>
virt: introduce get_test_entrypoint_func

This utility function warns about legacy and new style entry point
function names and returns the most suitable function.

It also keeps the logic (now a bit more elaborated) on determining
the most suitable function in a central place.

Some time from now, the so called legacy naming standard will be
deprecated for good and only the "run()" function name will be
allowed as a test entry point.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Merge pull request #1196 from clebergnu/subtest_run_method_name

virt: change test entry point method name