net: only advertise passt in netdev help when CONFIG_PASST

show_netdevs() lists each conditionally-compiled netdev backend behind
its CONFIG_* guard (CONFIG_SLIRP for "user", CONFIG_L2TPV3 for
"l2tpv3", CONFIG_NET_BRIDGE for "bridge", ...). The "passt" entry was
added unconditionally, so "-netdev help" advertises passt even in
builds configured with --disable-passt. Trying to use it then fails
with "Parameter 'type' does not accept value 'passt'", since the QAPI
NetClientDriver enum member "passt" is gated by 'if': 'CONFIG_PASST'.

Guard the help entry with CONFIG_PASST so the advertised backends match
those actually compiled into the binary.

Signed-off-by: Rohitashv Kumar <roohiit@amazon.de>
Reviewed-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>
Reviewed-by: Puranjay Mohan <puranjay@kernel.org>
Message-ID: <20260713194511.1058450-1-rohit.kuma1313@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>

net/net.c

@@ -1385,7 +1385,9 @@ void show_netdevs(void)
         "dgram",
         "hubport",
         "tap",
+#ifdef CONFIG_PASST
         "passt",
+#endif
 #ifdef CONFIG_SLIRP
         "user",
 #endif