Fix <cpu> element in domain XML schema

Fix <cpu> element in domain XML schema

The current schema is more permissive than the XML parsing code in
libvirt. Precisely, 'match' attribute is optional in schema while in
reality its use is bound to <model> element:

- <cpu> element without 'match' attribute is allowed only if <topology>
  element is the only child element of <cpu>

- <cpu> element with 'match' attribute requires <model> element to be
  present; <topology> and <feature> elements are optional

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>