qemu_domain: Introduce qemuDomainDiskSourceDiffers
    
    This new private API should return true iff sources of two disks
    differs in sense that qemu should be instructed to change the
    disk backend. For instance, ejecting a CDROM is such case, or
    pointing disk into a different ISO location, and so on.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>