lbvirtconnection: use root logger

Assign log variable the default logger which is the root logger.
The log of libvirtconnection is not configured in the configuration file
which cause the all the logs that using that not to be seen in the vdsm.log

The use of the root logger will make the log be seen in the vdsm.log

Change-Id: I8f1a7fd7e2dec9bb9ab037348f0276feb746b787
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/22907
Reviewed-by: Dan Kenigsberg <danken@redhat.com>