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: Ruda Moura <rmoura@redhat.com>
virttest: Test option '--log-level' through a list of fixed values.

Test possible values for option '--log-level' based on a list of fixed
log level names (stored in SUPPORTED_LOG_LEVELS).

Signed-off-by: Ruda Moura <rmoura@redhat.com>
Merge pull request #1067 from ruda/set-log-level

Add option '--log-level' to control log level for the top level log file