Add functions to setup user namespaced filesystems

When using user namespaces, the contents of the container's
file systems need to be owned by the targeted user on the host.
These functions will change the ownership of the files in the
filesystem to the proper owners.

For example, when mapping root in the container to user 500
on the host, the contents of the filesystem owned by root need
to be changed to be owned by user 500.

Partially implements: bp libvirt-lxc-user-namespaces

Change-Id: I78afc2686f17b5e5ec740a3d0a1963df7cc92755
Merge "Add functions to setup user namespaced filesystems"