[Libvir] Capabilities XML format

[Mark McLoughlin]

Hey,
	Just a few brief comments after using it:

  - <os_type>xen<os_type> is equivalent to <os><type>linux</type></os>, 
    which is pretty confusing. Sounds to me like we should deprecate 
    the latter and move to "xen" everywhere for this.

  - We have <host><arch>x86_64</arch></host> and 
    <guest><arch name="x86_64"/></guest>. I can see why we did that, 
    but it seems odd to me to have different syntaxes for <arch> 
    depending on its context.

  - I'm not sure how interesting the stuff in <guest><arch> is ... how 
    were we envisaging these bits being used?

  - It's a pain to have to parse XML to discover whether e.g. you can 
    run a 64 bit guest. I'd much prefer to see the kind of API I'm 
    using here:

      http://www.gnome.org/~markmc/code/test-caps.py

    e.g.

      caps = cnx.getCapabilities()
      supports_x86_64 = False
      for guest in caps.guests:
          if guest.arch == "x86_64":
              supports_x86_64 = True

Cheers,
Mark.