Add chardev option to disable signal.

If I am using vga and serial which is stdio and hit C-c on
serial console, qemu terminates. That is annoying for me.
So make it configurable whether signal is generated when C-c is hit.

Signed-off-by: Kusanagi Kouichi <slash@ac.auone-net.jp>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

qemu-config.c

@@ -134,6 +134,9 @@ QemuOptsList qemu_chardev_opts = {
         },{
             .name = "mux",
             .type = QEMU_OPT_BOOL,
+        },{
+            .name = "signal",
+            .type = QEMU_OPT_BOOL,
         },
         { /* end if list */ }
     },