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>