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: I3e439efde90ffbef0e2fb364bb826042e9f1f7da
Bug-Url: https://bugzilla.redhat.com/1006848
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>