Instead of globally keeping track of the free device ids we just
start from 0 each run and handle EEXIST error and try the next one.
This way we don't need any global state for the device ids, which
means we can read device metadata lazily. This is important for
multi-process use of the backend.
Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
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