Add a new 'vsock' element for the vsock device.
The 'model' attribute is optional.
A <source cid> subelement should be used to specify the guest cid,
or <source auto='yes'/> should be used.
https://bugzilla.redhat.com/show_bug.cgi?id=1291851
Signed-off-by: Ján Tomko <jtomko@redhat.com>
