This has bugged me for years – with random success depending on sudo, su – etc.

The proper solution:-

steve@studio:~$ ssh -X 192.168.0.201
Last login: Fri Feb 10 21:54:11 2017 from 192.168.0.247
[steve@fleabox ~]$ xauth list
fleabox.track3.org.uk/unix:12  MIT-MAGIC-COOKIE-1  b4339e07fb0e4febdde6128fc56419e4
[steve@fleabox ~]$ sudo su -
[sudo] password for steve: 
[root@fleabox ~]# xauth add fleabox.track3.org.uk/unix:12  MIT-MAGIC-COOKIE-1  b4339e07fb0e4febdde6128fc56419e4
[root@fleabox ~]# virt-manager &
[1] 7168

Success!