Acquire the per-handle lock on entering each public API function.
The lock is released by a cleanup handler, so we only need to use the
ACQUIRE_LOCK_FOR_CURRENT_SCOPE macro at the top of each function.
Note this means we require __attribute__((cleanup)). On platforms
where this is not supported, the code will probably hang whenever a
libguestfs function is called.
The only definitive list of public APIs is found indirectly in the
generator (in generator/c.ml : globals).
b9847b404c