qemu: Support passing custom options to setup user mode networking

This can be used to specify things like port forwards, TFTP server,
etc.

qemu/Config.in

@@ -61,6 +61,12 @@ config QEMU_NET_BRIDGE_DEV
 	depends on QEMU_NET_BRIDGE
 	default "virbr0"
 
+config QEMU_NET_USER_OPTS
+	string "User mode options"
+	depends on QEMU_NET_USER
+	help
+	  Extra -nic user,<OPTIONS>
+
 config QEMU_NET_TAP_N
 	int "Number of TAPs"
 	depends on QEMU_NET_TAP

qemu/qemu.sh

@@ -75,7 +75,9 @@ net_args()
 	    echo -n "-nic tap,ifname=qtap$i,script=no,model=$QEMU_NET_MODEL "
 	done
     elif [ "$QEMU_NET_USER" = "y" ]; then
-	echo -n "-nic user,model=$QEMU_NET_MODEL "
+	[ "$QEMU_NET_USER_OPTS" ] && QEMU_NET_USER_OPTS="$QEMU_NET_USER_OPTS,"
+
+	echo -n "-nic user,${QEMU_NET_USER_OPTS}model=$QEMU_NET_MODEL "
     else
 	echo -n "-nic none"
     fi