qapi: add the QKeyCode enum key_defs[] in monitor.c is a mapping table of keys and keycodes, this patch added a QKeyCode enum. Key's index in the enmu is same as keycode's index in key_defs[]. Signed-off-by: Amos Kong <akong@redhat.com> Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>