qxl: move qxl_send_monitors_config()

Needed to avoid a forward declaration in a followup patch.
Pure code move, no functional change.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20180420071904.24276-3-kraxel@redhat.com