Consider the following XML snippet:

    <memory model=''>
      <target>
        <size unit='KiB'>523264</size>
        <node>0</node>
      </target>
    </memory>

Whats wrong you ask? The @model attribute. This should result in
an error thrown into users faces during virDomainDefine phase.
Except it doesn't. The XML validation catches this error, but if
users chose to ignore that, they will end up with invalid XML.
Well, they won't be able to start the machine - that's when error
is produced currently. But it would be nice if we could catch the
error like this earlier.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>