We should not be exposing the store outside of Libpod. We want to
encapsulate it as an internal implementation detail - there's no
reason functions outside of Libpod should directly be
manipulating container storage. Convert the last use to invoke a
method on Libpod instead, and remove the function.
[NO TESTS NEEDED] as this is just a refactor.
Signed-off-by: Matthew Heon <mheon@redhat.com>