hw/input/hid: support alternative sysrq/break scancodes for gtk-vnc

The printscreen/sysrq and pause/break keys currently don't work for guests
using -usbdevice keyboard when accessed through vnc with a gtk-vnc based
client.

The reason for this is a mismatch between gtk-vnc and qemu in how these keys
should be mapped to XT keycodes.

On the original IBM XT these keys behaved differently than other keys.

Quoting from https://www.win.tue.nl/~aeb/linux/kbd/scancodes-1.html:

The keys PrtSc/SysRq and Pause/Break are special. The former produces
scancode e0 2a e0 37 when no modifier key is pressed simultaneously, e0 37
together with Shift or Ctrl, but 54 together with (left or right) Alt.  (And
one gets the expected sequences upon release.  But see below.) The latter
produces scancode sequence e1 1d 45 e1 9d c5 when pressed (without modifier)
and nothing at all upon release.  However, together with (left or right)
Ctrl, one gets e0 46 e0 c6, and again nothing at release.  It does not
repeat.

Gtk-vnc supports the 'QEMU Extended Key Event Message' RFB extension to send
raw XT keycodes directly to qemu, but the specification doesn't explicitly
specify how to map such long/complicated keycode sequences.  From the spec
(https://github.com/rfbproto/rfbproto/blob/master/rfbproto.rst#qemu-extended-key-event-message)

The keycode is the XT keycode that produced the keysym. An XT keycode is an
XT make scancode sequence encoded to fit in a single U32 quantity.  Single
byte XT scancodes with a byte value less than 0x7f are encoded as is.
2-byte XT scancodes whose first byte is 0xe0 and second byte is less than
0x7f are encoded with the high bit of the first byte set

hid.c currently expects the keycode sequence with shift/ctl for sysrq (e0 37
-> 0xb7 in RFB), whereas gtk-vnc uses the sequence with alt (0x54).
Likewise, hid.c expects the code without modifiers (e1 1d 45 -> 0xc5 in
RFB), whereas gtk-vnc sends the keycode sequence with ctrl for pause (e0 46
-> 0xc6 in RFB).

See keymaps.cvs in gtk-vnc for the mapping used:
https://git.gnome.org/browse/gtk-vnc/tree/src/keymaps.csv#n150

Now, it isn't obvious to me which sequence is really "right", but as the
0x54/0xc6 keycodes are currently unused in hid.c, supporting both seems like
the pragmatic solution to me.  The USB HID keyboard boot protocol used by
hid.c doesn't have any other mapping applicable to these keys.

The other guest keyboard interfaces (ps/2, virtio, ..) are not affected,
because they handle these keys differently.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Message-id: 20161028145132.1702-1-peter@korsgaard.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>