vdsm-reg: avoid issuing errors to console

Using logging.error() during the ovirt-node dialog results in writing
messages to console. This is undesired in dialog mode.

Change the error to debug to avoid.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=947905
Change-Id: Ib25f516b74dc03cca4fe696467121c8e8db06b76
Signed-off-by: Alon Bar-Lev <alonbl@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/13561
Reviewed-by: Dan Kenigsberg <danken@redhat.com>