qemu: Provide default LUN=0 for iSCSI if not provided

qemu: Provide default LUN=0 for iSCSI if not provided

https://bugzilla.redhat.com/show_bug.cgi?id=1477880

If the "/#" is missing from the provided iSCSI path, then we need
to provide the default LUN of /0; otherwise, QEMU will fail to parse
the URL causing a failure to either create the guest or hotplug
attach the storage.

During post parse, for any iSCSI disk or hostdev, scan the source
path looking for the presence of '/', if found, then we can assume
the LUN is provided.  If not found, alter the input XML to add the
"/0".  This will cause the generated XML to have the generated
value when the domain config is saved after post parse.