CP-9441: debugging improvements

Option to make tapdisk not daemonise itself, this makes debugging with
valgrind much easier.

Also, documentation on how to debug tapdisk with Valgrind.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
I don't see why I need to explain in the README how a specific option is used, we don't do this for other blktap options since they're self-explanatory.