Validating the vcpu count is more intricate and doing it in the XML
parser will make previously valid configs (with older qemus) vanish.

Now that we have a very similar check in the qemu domain validation
callback we can do it in a more appropriate place.

This basically reverts commit b54de083.

Partially resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1370066