minor changes in libvirt connection

change logging levels and use functools.wraps

Change-Id: I5b4b1ddea998adc3003255593dc139ca4321cbd0
Signed-off-by: Mooli Tayer <mtayer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/19463
Reviewed-by: Saggi Mizrahi <smizrahi@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>