Set correct SELinux context options on container & VM mounts

[stgraber]

Per https://discuss.linuxcontainers.org/t/unable-to-add-lxc-rootfs-mount-options-context/20943/14 we should try and at least set the correct context mount options whenever we mount a container or VM data directory.

Ideally we'd have something more robust where we can handle per-instance labeling but that would require someone with more SELinux knowledge than I have to get this done properly, put testing in place and keep an eye on it so it doesn't regress :)

So for now, we'll try to go for low hanging fruits and just make things better where feasible.