conf: domain: reject name containing '/'

conf: domain: reject name containing '/'

Trying to define a domain name containing an embedded '/'
will immediately fail when trying to write the XML to disk for
our stateful drivers. This patch explicitly rejects names
containing a '/', and provides an xmlopt feature for drivers
to avoid this validation check, which is enabled in every
non-stateful driver that already has xmlopt handling wired up.

(Technically this could reject a previously accepted vmname like
 '/foo', however at least for the qemu driver that falls over
 later when starting qemu)

https://bugzilla.redhat.com/show_bug.cgi?id=639923