ps2: fix scancodes sent for Shift/Ctrl+Print key combination

The 'Print' key is special in the AT set 1 / set 2 scancode definitions.

An unmodified 'Print' key is supposed to send

 AT Set 1:  e0 2a e0 37 (Down)  e0 b7 e0 aa (Up)
 AT Set 2:  e0 12 e0 7c (Down)  e0 f0 7c e0 f0 12 (Up)

which QEMU gets right. When combined with Shift/Ctrl (both left and right
variants), the leading two bytes should be dropped, resulting in

 AT Set 1:  e0 37 (Down)  e0 b7 (Up)
 AT Set 2:  e0 7c (Down)  e0 f0 7c (Up)

This difference is pretty benign, since of all the operating systems I have
checked (Linux, FreeBSD and OpenStack), none bother to check the leading two
bytes anyway. This change none the less makes the ps2 device better follow real
hardware behaviour.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 20171019142848.572-6-berrange@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
