conf: Don't validate vcpu count in XML parser
    
    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 b54de0830a.
    
    Partially resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1370066