Make getVlanDevice use the faster netlink information

getVlanId was already using the newer and faster netlink information
while getVlanDevice had remained creating a process and parsing the
output. This patch makes it more consistent and faster.

Change-Id: Ib5ab4fd8fa454770669cbc1f8428e80cd70ec66c
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/24989
Reviewed-by: Dan Kenigsberg <danken@redhat.com>