unshare-qemuwrapper: Use free port

Benjamin Drungrequested to merge
bdrung/sbuild:configurable-port into main

The QEMU autopkgtest runner also uses the local port 10022. So search for a free port in unshare-qemuwrapper. Fall back to port 10022 in case python3 is not available.

Edited by Benjamin Drung

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