Set gpu_mode with guest_swiftshader in snapshot_test.

- gpu_mode defaults to `auto`, to snapshot a device gpu_mode must be guest_swiftshader.

Author: ser-io

e2etests/orchestration/snapshot_test/main_test.go

@@ -132,6 +132,9 @@ func createDevice(srv hoclient.HostOrchestratorClient, group_name, hostPkgDir, i
         "disk": {
           "default_build": "@image_dirs/` + imageDir + `"
         },
+        "graphics": {
+          "gpu_mode": "guest_swiftshader"
+        },
         "streaming": {
           "device_id": "cvd-1"
         }