vdsm: Don't log irrelevant errors with level ERROR

libvirtconnection.get() wrapMethod logs unhandled libvirtErrors
as ERROR. However it's irrelevant at that point, therefore it
should log it only as DEBUG message.

Change-Id: I62c395734173a996bc8237d24b85f1bb32a0be35
Bug-Url: https://bugzilla.redhat.com/955593
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/14226
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>