hw/qxl: exit on failure to register qxl interface

This prevents a segfault later on when the device reset handler
tries to access a NULL ssd.worker since interface_attach_worker has
not been called.

Signed-off-by: Alon Levy <alevy@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

hw/qxl.c

diff --git a/hw/qxl.c b/hw/qxl.c
index eb7707c..e99ed55 100644
--- a/hw/qxl.c
+++ b/hw/qxl.c
@@ -2037,7 +2037,11 @@
 
     qxl->ssd.qxl.base.sif = &qxl_interface.base;
     qxl->ssd.qxl.id = qxl->id;
-    qemu_spice_add_interface(&qxl->ssd.qxl.base);
+    if (qemu_spice_add_interface(&qxl->ssd.qxl.base) != 0) {
+        error_report("qxl interface %d.%d not supported by spice-server\n",
+                     SPICE_INTERFACE_QXL_MAJOR, SPICE_INTERFACE_QXL_MINOR);
+        return -1;
+    }
     qemu_add_vm_change_state_handler(qxl_vm_change_state_handler, qxl);
 
     init_pipe_signaling(qxl);