Looks like we had our own copy of the "getDevices" code already, so use
that code (which also matches the code that's used to _generate_ the spec,
so a better match).
Moving the code to a separate file, I also noticed that the _unix and _linux
code was _exactly_ the same (baring some `//nolint:` comments), so also
removing the duplicated code.
With this patch applied, we removed the dependency on the libcontainer/devices
package (leaving only libcontainer/user).
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
