Add support for SIDE and EXTRA buttons.

The constants for buttons in both SPICE and QEMU are defined as
  LEFT
  MIDDLE
  RIGHT
  UP
  DOWN
  SIDE
  EXTRA
(same order).

"button_mask" contains for each bit the state of a button. Qemu currently
uses bits 0, 1, 2 respectively as LEFT, RIGHT, MIDDLE; also add bits 4
and 5 as UP and DOWN (using wheel movements). SPICE protocol uses
a bitmask based on the order above where LEFT is bit 0, MIDDLE is
bit 1 and so on till EXTRA being bit 6. To avoid clash with Qemu usage
SPICE bitmask from SIDE are move a bit more resulting respectively
in 0x40 and 0x80 values.

Signed-off-by: Frediano Ziglio <freddy77@gmail.com>
Message-id: 20200820145851.50846-1-fziglio@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>