Feature/kvm move to json in xml

Moving just KVM SR to json-in-xml serialization.

Blockers:

• SlapOS Master supports (does not destroy) incorrect serialization in the instance parameters (xml sent in case of json-in-xml and vice-versa)
• SlapOS Master renders connection xml correctly if slapos.cookbook:publish.serialised is used
• https://lab.nexedi.com/nexedi/slapos/merge_requests/1174
• https://lab.nexedi.com/nexedi/slapos/merge_requests/1176