netinfo: Retrieve a VLAN's NIC after its ID in getVlanBondingNic

Obviously, we cannot supply a VLAN's underlying device (eth0) to getVlanID,
it has to be the VLAN (eth0.100) itself.

Change-Id: Iae4c4ed1dc4e72451a6187d8a8c6ae01dafe1aed
Fixes: http://gerrit.ovirt.org/18182
Signed-off-by: Ondřej Svoboda <osvoboda@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/36193
Reviewed-by: Ido Barkan <ibarkan@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>