chardev/socket: print a more correct command-line address

Better reflect the command line version of the socket address arguments,
following the now recommended long-form opt=on syntax.

Complement/fixes commit 9d902d51 "chardev: do not use short form boolean
options in non-QemuOpts character device descriptions".

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>

chardev/char-socket.c

@@ -468,9 +468,9 @@ static char *qemu_chr_socket_address(SocketChardev *s, const char *prefix)
 
 #ifdef CONFIG_LINUX
         if (sa->has_abstract && sa->abstract) {
-            abstract = ",abstract";
+            abstract = ",abstract=on";
             if (sa->has_tight && sa->tight) {
-                tight = ",tight";
+                tight = ",tight=on";
             }
         }
 #endif