virttest: Add parameter to control log level.

Initial effort to add the parameter "--log-level" when running tests,
to control the log level. For example:

./run -t qemu --log-level=error

Will only log ERROR messages.

Note that The default value remains DEBUG level.
Currently it only applies to log/debug.log.
Signed-off-by: NRuda Moura <rmoura@redhat.com>