log: write only errors on stderr

Previously the logging system wrote all the messages
to the standard error.

Now only errors are logged to the standard error, all the
other message are logged to standard output.

Change-Id: I4e6cde74421c5b513cba0c2b56fa73acfa212e87
Bug-Url: https://bugzilla.redhat.com/1006792
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>