util/oslib-win32: Do not treat null @errp as &error_warn

util/oslib-win32: Do not treat null @errp as &error_warn

qemu_socket_select() and its wrapper qemu_socket_unselect() treat a
null @errp as &error_warn.  This is wildly inappropriate.  A caller
passing null @errp specifies that errors are to be ignored.  If
warnings are wanted, the caller must pass &error_warn.

Change callers to do that, and drop the inappropriate treatment of
null @errp.

This assumes that warnings are wanted.  I'm not familiar with the
calling code, so I can't say whether it will work when the socket is
invalid, or WSAEventSelect() fails.  If it doesn't, then this should
be an error instead of a warning.  Invalid socket might even be a
programming error.

These warnings were introduced in commit f5fd677ae7cf (win32/socket:
introduce qemu_socket_select() helper).  I considered reverting to
silence, but Daniel Berrangé asked for the warnings to be preserved.

Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20250923091000.3180122-9-armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>