qemu: migration: Forbid 'nbd' migration of non-shared storage if TLS is requested
    
    Since libvirt is currently not able to setup the NBD migration stream
    secured by TLS we should not allow such migration since data would be
    transferred unencrypted.
    
    This will break compatibility of TLS migration if non-shared storage is
    requested but the security implications are more severe.
    
    Signed-off-by: Peter Krempa <pkrempa@redhat.com>