Next vz id will consider vz directories as well

The next vz id to use to create a new container should also consider
the vz id directories that exist in addition to containers that are
running. There can be container that was deleted but for some reason
did not clean up all its directories within /var/lib/vz/{private,root}
which will cause the next container that reuses that id to fail.

Change-Id: I5813c30f594d99a6a21054cd5b3caa24d85039fe
Merge "Next vz id will consider vz directories as well"