Fix --volumes-from mount failure

As explained in https://github.com/dotcloud/docker/issues/4979
--volumes-from fails with ENOFILE errors.

This is because the code tries to look at the "from" volume without
ensuring that it is mounted yet. We fix this by mounting the containers
before stating in it.

Also includes a regression test.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)