vnc: fix numlock+capslock tracking

vnc: fix numlock+capslock tracking

This patch makes the numlock+capslock tracking logic only look at
keydown events.  Without this patch the vnc server will insert
bogous capslock keypress in case it sees the following key sequence:

  shift down --- 'A' down --- shift up  --- 'A' up
                                         ^ here

It doesn't hurt with a PS/2 keyboard, but it disturbs the USB Keyboard.
And with the key event queue just added to the usb keyboard the guest
will actually notice.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>