fixing race while sampling interfaces

The race condition is that a vlan device is reported by getLinks and then
it is no longer there by the time InterfaceSample is called

Bug-Url: https://bugzilla.redhat.com/1173929

Change-Id: I059a5bfa4c140d88f512dc9a51d18f02956710de
Signed-off-by: ibarkan <ibarkan@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/36138
Reviewed-by: Dan Kenigsberg <danken@redhat.com>