vcpu: add current attribute to <vcpu> element

vcpu: add current attribute to <vcpu> element

Syntax agreed on in
https://www.redhat.com/archives/libvir-list/2010-September/msg00476.html

<domain ...>
  <vcpu current='x'>y</vcpu>
...

can now be used to specify 1 <= x <= y current vcpus, in relation
to the boot-time max of y vcpus.  If current is omitted, then
current and max are assumed to be the same value.

* docs/schemas/domain.rng: Add new attribute.
* docs/formatdomain.html.in: Document it.
* tests/qemuxml2argvdata/qemuxml2argv-smp.xml: Add to
domainschematest.
* tests/xml2sexprdata/xml2sexpr-pv-vcpus.xml: Likewise.