conf: network: reject name containing '/'

conf: network: reject name containing '/'

Trying to define a network name containing an embedded '/'
will immediately fail when trying to write the XML to disk.
This patch explicitly rejects names containing a '/'

Besides the network bridge driver, the only other network
implementation is a very thin one for virtualbox, which seems to
use the network name as a host interface name, which won't
accept '/' anyways, so I think this is fine to do unconitionally.

https://bugzilla.redhat.com/show_bug.cgi?id=787604