vfio-user: set up container access to the proxy

vfio-user: set up container access to the proxy

The user container will shortly need access to the underlying vfio-user
proxy; set this up.

Originally-by: John Johnson <john.g.johnson@oracle.com>
Signed-off-by: Elena Ufimtseva <elena.ufimtseva@oracle.com>
Signed-off-by: Jagannathan Raman <jag.raman@oracle.com>
Signed-off-by: John Levon <john.levon@nutanix.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250625193012.2316242-12-john.levon@nutanix.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>