[PATCH] qxl: Call spice_qxl_driver_unload from qxl_enter_vg

With a SPICE_DISPLAY_CAP_MONITORS_CONFIG capable client, the client needs to
know what part of the primary to use for each monitor. If the guest driver
does not support this, the server sends messages to the client for a
single monitor spanning the entire primary.

As soon as the guest calls spice_qxl_monitors_config_async once, the server
sets the red_worker driver_has_monitors_config flag and stops doing this.

This is a problem when the driver gets unloaded, for example after a reboot
or when switching to a text vc with usermode mode-setting under Linux.

To reproduce this start a multi-mon capable Linux guest which uses
usermode mode-setting and then once X has started switch to a text vc. Note
how the client window does not only not resize, if you try to resize it
manually you always keep blackborders since the aspect is wrong.

This patch calls a new spice-server method called spice_qxl_driver_unload
which clears the driver_has_monitors_config flag inside the server, thereby
fixing this.

http://lists.gnu.org/archive/html/qemu-devel/2013-04/msg04502.html