conf: Catch invalid memory model earlier
    
    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: Michal Privoznik <mprivozn@redhat.com>