`muvm --privileged` should have its own XDG_RUNTIME_DIR

[kitlith]

Keeping in mind #183

kitlith@quillian:~$ muvm -i -- sh -c 'id; echo $XDG_RUNTIME_DIR'
uid=1000(kitlith) gid=1000(kitlith) groups=1000(kitlith)
/tmp/muvm-run-1000-YOz47f

kitlith@quillian:~$ muvm -i --privileged -- sh -c 'id; echo $XDG_RUNTIME_DIR'
uid=0(root) gid=0(root) groups=0(root)
/tmp/muvm-run-1000-YOz47f

I would instead expect there to be a /tmp/muvm-run-0-<whatever> when running as root inside the VM.

[kitlith]

mm, this is impacted by the whole filesystem permissions thing. Some software checks that XDG_RUNTIME_DIR is owned by root, and in (host) tmpfs, we can't create root-owned files. This can be worked around by mounting a guest tmpfs at, say, /tmp/muvm-run-<whatever> and creating /tmp/muvm-run-<whatever>/0.

[kitlith]

actually, slightly off topic, is there a reason why we're using /tmp/... instead of /var/run/user/{id} under muvm?