devices: vhost-user: Add a test case for ClientExit
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 <50abc1d85bedfd527fc7dee23f4819d1f0d946ec@google.com>
Reviewed-by: Alexandre Courbot <301cd6a3ea50a894b4b51e385c5eb12b9a4ffc0c@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Commit-Queue: Keiichi Watanabe <815e4e8f7c92a23424d7c3f51ead622bcedbc720@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm