virDomainCreateXML: Make domain definition transient

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>