devices: vhost-user: Add a test case for ClientExit

The vhost-user VMM process is supposed to stop before the device
process, and the device process is supposed to get
`vmm_vhost::Error::ClientExit` when the VMM stops. Add the check in the
existing unit test.

BUG=b:219674197
BUG=b:220639724
TEST=cargo test in /devices/

Change-Id: I1768b9b43855e3b80f1e6e175877317338309e8a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3543005
Reviewed-by: Vikram Auradkar <auradkar@google.com>
Reviewed-by: Alexandre Courbot <acourbot@chromium.org>
Tested-by: kokoro <noreply+kokoro@google.com>
Commit-Queue: Keiichi Watanabe <keiichiw@chromium.org>

devices/src/virtio/vhost/user/device/handler.rs

@@ -997,6 +997,9 @@ mod tests {
                     .unwrap();
             }
 
+            // The VMM side is supposed to stop before the device side.
+            drop(vmm_handler);
+
             vmm_bar.wait();
         });
 
@@ -1035,5 +1038,10 @@ mod tests {
         }
 
         dev_bar.wait();
+
+        match listener.handle_request() {
+            Err(VhostError::ClientExit) => (),
+            r => panic!("Err(ClientExit) was expected but {:?}", r),
+        }
     }
 }