virDomainCreateXML: Make domain definition transient
    
    https://bugzilla.redhat.com/show_bug.cgi?id=871452
    
    So, you want to create a domain from XML. The domain already
    exists in libvirt's database of domains. It's okay, because name
    and UUID matches. However, on domain startup, internal
    representation of the domain is overwritten with your XML even
    though we claim that the XML you've provided is a transient one.
    The bug is to be found across nearly all the drivers.
    Le sigh.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>