dummies and veths with changeable prefix

Veth's and dummy's create() functions have unrequired attribute
prefix.

Change-Id: I7f8bd58abd5cbf73ad69b0e668d1dde83d247d05
Signed-off-by: Petr Horáček <phoracek@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/31164
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>