Use 'ip link' to get different kinds of interfaces

It's a little faster than listing the dir '/sys/class/net'

Change-Id: Iecc2300963fe1834defb99f5be92b25c0218cf00
Signed-off-by: Mark Wu <wudxw@linux.vnet.ibm.com>
Reviewed-on: http://gerrit.ovirt.org/13668
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>