cputune: allow interleaved xml

cputune: allow interleaved xml

I noticed this with the recent iothread pinning code, but the
problem existed longer than that. The XML validation required
users to supply <cputune> children in a strict order, even though
there was no conceptual reason why they can't occur in any order.

docs/ changes best viewed with -w

* docs/schemas/domaincommon.rng (cputune): Add interleave.
* tests/qemuxml2argvdata/qemuxml2argv-cputune-iothreads.xml: Swap
up order, copying canonical form...
* tests/qemuxml2xmloutdata/qemuxml2xmlout-cputune-iothreads.xml:
...here.
* tests/qemuxml2xmltest.c (mymain): Mark the difference.

Signed-off-by: Eric Blake <eblake@redhat.com>