netconf/ifcfg: remove only backup files

clearBackups() removes the entire directory /var/lib/vdsm/netconfback/.
Because of that, TPS test (rpm -V vdsm) has failed.

This patch, will remove all files/directoies inside /var/lib/vdsm/netconfback/
and keep this directory.

Change-Id: Ie0144aef836011f5f3abf35086d349e25ca83683
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=958009
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/14599
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>