ui/gtk: Fix non-working DELETE key

GTK generates key events for the delete key with key->string[0] = 0x7f
... but this does not work right with the readline_handle_byte()
function in util/readline.c, since this treats the keycode 127 as
backspace. So let's add a special case for the GTK delete key to make
this key behave right in the monitor interface of the GTK ui.

Buglink: https://bugs.launchpad.net/qemu/+bug/1619438


Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-id: 1477570647-7100-1-git-send-email-thuth@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>