Use slirp_pollfds_fill_socket on libslirp >= 4.9.0

[archigup]

Description

slirp_pollfds_fill() and its int-based SlirpAddPollCb callback were deprecated in libslirp 4.9.0 (2025-01-30) in favour of slirp_pollfds_fill_socket() and SlirpAddPollSocketCb. The new API carries the descriptor as a slirp_os_socket — an int on POSIX, but a pointer-wide SOCKET on Win64 — which is why libslirp introduced it (the old int callback truncates socket handles on Windows).

Building MBuffNetifBackendLibslirp.c against libslirp >= 4.9.0 therefore produces:

MBuffNetifBackendLibslirp.c: In function 'vReceiveThread':
MBuffNetifBackendLibslirp.c:709:13: warning: 'slirp_pollfds_fill' is deprecated [-Wdeprecated-declarations]

This change guards the poll-fill call and the add-poll callback signature on SLIRP_CHECK_VERSION( 4U, 9U, 0U ) (matching the version-guard style already used throughout this file), so:

• libslirp >= 4.9.0 uses slirp_pollfds_fill_socket() with a slirp_os_socket callback (no warning, correct on Win64),
• libslirp < 4.9.0 keeps the existing slirp_pollfds_fill() path unchanged.

The callback body is unchanged — it stores the descriptor into struct pollfd.fd (already a SOCKET inside WSAPOLLFD on Windows), so no truncation occurs on either platform.

Fixes #1325

Test Steps

Compiled MBuffNetifBackendLibslirp.c against real libslirp 4.9.3 headers and an emulated 4.8.0 header set:

• Before (4.9.3): 1 deprecation warning at the slirp_pollfds_fill call.
• After (4.9.3): compiles clean with -Werror=deprecated-declarations; 0 deprecation warnings. Preprocessor selects slirp_pollfds_fill_socket + the slirp_os_socket callback.
• After (emulated 4.8.0): preprocessor selects the legacy slirp_pollfds_fill + int callback and compiles successfully, confirming the #else path is preserved for older libslirp.

Checklist:

• I have tested my changes. No regression in existing tests.
• I have modified and/or added unit-tests to cover the code changes in this Pull Request. (N/A — build-time/conditional-compilation change against a host library; verified by compiling the backend against libslirp >= 4.9.0 and < 4.9.0.)
• I have updated Long-Term-Support related files if required.
• I have run the formatting tool on this PR.

Related Issue

#1325

By submitting this pull request, I confirm that my contribution is made under the terms of the MIT license.

[kstribrnAmzn]

Looks good to me. Maybe sometime in the distant future we can deprecate this check, but I understand keeping it for current users.