netdev-dpdk: Do not add vhost-user ports with '/' or '\' in name.

netdev-dpdk: Do not add vhost-user ports with '/' or '\' in name.

This check prevents an obvious way for a vhost-user socket to escape the
intended directory.

There might be other ways to escape the directory (none comes to mind at
the moment), but this is a problem that should be properly solved by
mandatory access control.

A similar check is done for a bridge name, since that name is used as
part of a socket as well.

Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Acked-by: Flavio Leitner <fbl@sysclose.org>