net/slirp: Clean up error reporting

net/slirp: Clean up error reporting

net_slirp_register_poll_sock() and net_slirp_unregister_poll_sock()
report WSAEventSelect() failure with error_setg(&error_warn, ...).
error_setg_win32(&error_warn, ...) is undesirable just like
error_setg(&error_fatal, ...) and error_setg(&error_abort, ...)  are.
Replace by warn_report().

The failures should probably be errors, but these functions implement
callbacks that cannot fail, exit(1) would be too harsh, and silent
failure we don't want.  Thus, warnings.

Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-ID: <20250923091000.3180122-7-armbru@redhat.com>
Reviewed-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>