swig: guard against silent attribute typos on SWIG objects

[martin-belanger]

Problem

SWIG-generated Python classes allow assigning arbitrary attributes on instances. A caller writing:

ctrl.dhchap_key = x        # typo — correct name is dhchap_ctrl_key

silently creates a new Python instance attribute instead of calling the C-level property setter. The value is discarded and the bug is invisible at runtime. This has already caused a real bug in nvme-stas where dhchap_key was written instead of dhchap_ctrl_key, meaning the controller DHCHAP key was never actually set. What actually happened is that the property dhchap_key had been renamed to dhchap_ctrl_key in libnvme's python bindings, but had not been changed in nvme-stas causing a silent failure.

The same silent-failure applies to the ctrl constructor dict: a typo in a key such as subsysnqn_typo was previously ignored without error.

Changes

libnvme/libnvme/nvme.i

Add a __setattr__ guard (via %pythoncode) to four classes:
libnvme_global_ctx, libnvme_host, libnvme_subsystem, and libnvme_ctrl.

The guard:

• Passes through this (SWIG's raw C pointer) and _-prefixed names (GC-anchoring refs set by %pythonappend blocks such as self.__host) via object.__setattr__, bypassing property lookup.
• Walks the MRO to find a property with an fset and calls it directly, equivalent to Python's default machinery but without allowing fallback to instance-dict creation.
• Raises AttributeError for everything else — covering both unknown names and %immutable properties (which have no fset).

Note: thisown has an fset, so it is handled correctly by the MRO walk without any special case.

In set_fctx_from_dict(), replace the silent fall-through at the end of the key-dispatch while loop with a PyErr_Format(PyExc_KeyError) - return -1. Every known key already hits continue; any key that falls through is therefore unknown. This avoids a second pass over the dict.

libnvme/libnvme/tests/test-setattr.py (new file)

Six test cases:

• Valid writable property assignment does not raise.
• Typo on a property name raises AttributeError immediately.
• Assignment to a %immutable (read-only) property raises AttributeError.
• Completely unknown attribute name raises AttributeError.
• Unknown key in the constructor dict raises KeyError.
• Missing required key (subsysnqn or transport) raises KeyError.

Testing

meson test -C .build

[igaw]

Thanks!