qxl+vnc: register a vm state change handler for dummy spice_server

When qxl + vnc are used, a dummy spice_server is initialized.
The spice_server has to be told when the VM runstate changes,
which is what this patch does.

Without it, from qxl_send_events(), the following error message is shown:
  qxl_send_events: spice-server bug: guest stopped, ignoring

Cc: qemu-stable@nongnu.org
Signed-off-by: Uri Lublin <uril@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
