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

src/vbox/vbox_common.c

@@ -273,6 +273,7 @@ static virDomainDefParserConfig vboxDomainDefParserConfig = {
     .macPrefix = { 0x08, 0x00, 0x27 },
     .devicesPostParseCallback = vboxDomainDeviceDefPostParse,
     .domainPostParseCallback = vboxDomainDefPostParse,
+    .features = VIR_DOMAIN_DEF_FEATURE_NAME_SLASH,
 };
 
 static virDomainXMLOptionPtr