There is an existing wrapper for unix.Unmount(..., MNT_DETACH) in
util_linux.go but that filters all errors and for volumes, we only want
to filter EINVAL. The existing libpod.Unmount seems to only have one
call site so perhaps these can be merged.
[NO NEW TESTS NEEDED]
Signed-off-by: Doug Rabson <dfr@rabson.org>