hooks: Extract devName function for openstacknet

Extracted function to calculate the device name used for OpenStack
Networking devices.

Change-Id: I8ece3bb16ac5b9bb8b28c26f2e96d2d74defd4dc
Signed-off-by: Mike Kolesnik <mkolesni@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/22584
Reviewed-by: Dan Kenigsberg <danken@redhat.com>