qemu: s/virDomainDiskDiffersSourceOnly/qemuDomainDiskChangeSupported/
    
    I always felt like this function is qemu specific rather than
    libvirt-wide. Other drivers may act differently on virDomainDef
    change and in fact may require talking to underlying hypervisor
    even if something else's than disk->src has changed.  I know that
    the function is still incomplete, but lets break that into two
    commits that are easier to review. This one is pure code
    movement.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>