This patch updates the vmmouse handler registration and activation.

Old behavior:
  vmmouse_read_id, vmmouse_request_relative and vmmouse_request_absolute
  unregister the handler and re-register it.

New behavior:
  vmmouse_request_relative and vmmouse_request_absolute will unregister
  the handler in case the mode did change.  Then register and active the
  handler with current mode if needed.

Note that the old code never ever *activates* the handler, so the
vmmouse doesn't receive events.  This trips up Fedora 14 for example:
Boot a default install without usb tablet, watch the X-Server activating
the vmmouse then, enjoy a non-functional mouse.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>