ps2: fix sending of PAUSE/BREAK scancodes

The processing of the scancodes for PAUSE/BREAK  has been broken since
the conversion to qcodes in:

  commit 8c10e0ba
  Author: Hervé Poussineau <hpoussin@reactos.org>
  Date:   Thu Sep 15 22:06:26 2016 +0200

    ps2: use QEMU qcodes instead of scancodes

When using a VNC client, with the raw scancode extension, the client
will send a scancode of 0xc6 for both PAUSE and BREAK. There is mistakenly
no entry in the qcode_to_number table for this scancode, so
ps2_keyboard_event() just generates a log message and discards the
scancode

When using a SPICE client, it will also send 0xc6 for BREAK, but
will send 0xe1 0x1d 0x45 0xe1 0x9d 0xc5 for PAUSE. There is no
entry in the qcode_to_number table for the scancode 0xe1 because
it is a special XT keyboard prefix not mapping to any QKeyCode.
Again ps2_keyboard_event() just generates a log message and discards
the scancode. The following 0x1d, 0x45, 0x9d, 0xc5 scancodes get
handled correctly. Rather than trying to handle 3 byte sequences
of scancodes in the PS/2 driver, special case the SPICE input
code so that it captures the 3 byte pause sequence and turns it
into a Pause QKeyCode.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 20170727113243.23991-1-berrange@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>