libvirt: define XML schema for recording nova instance metadata

Define an XML schema to be used in the libvirt guest config as
a metadata block to provide information about the instance
that is being run. When querying the libvirt guest XML this
is visible thus:

<domain type='kvm' id='6'>
  <name>instance-00000004</name>
  <uuid>a173eb64-f8e6-419b-a2b7-2d3283ecb26c</uuid>
  <metadata>
    <nova:instance xmlns:nova="http://openstack.org/xmlns/libvirt/nova/1.0">
      <nova:package version="2014.2"/>
      <nova:name>demo</nova:name>
      <nova:creationTime>2014-06-27 11:22:25+GMT</nova:creationTime>
      <nova:flavor name="m1.small">
        <nova:memory>2048</nova:memory>
        <nova:disk>20</nova:disk>
        <nova:swap>0</nova:swap>
        <nova:vcpus>1</nova:vcpus>
      </nova:flavor>
      <nova:owner>
        <nova:user uuid="18509018d00743c89518135e476003e0">admin</nova:user>
        <nova:project uuid="38bd8e984a3e435a87e950a441ee55aa">admin</nova:project>
      </nova:owner>
      <nova:root type="image" uuid="f0f0d38c-0a5f-4c54-83e5-d48006b888a7"/>
    </nova:instance>
  </metadata>
  ...snip...
</domain>

Blueprint: libvirt-driver-domain-metadata
Change-Id: I19432cb93bc1471118d9d323c712cdd9756a5926
Merge "libvirt: define XML schema for recording nova instance metadata"