devices: vvu: only notify txq after sending
devices: vvu: only notify txq after sending

Only signal the txq_notifier when a message is actually written.

BUG=None
TEST=launch ManaTEE sibling

Change-Id: I631720b270a75a010bac5efc94158ab1875dd99e
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/3787935
Reviewed-by: Alexandre Courbot <301cd6a3ea50a894b4b51e385c5eb12b9a4ffc0c@chromium.org>
Tested-by: David Stevens <2f87ba95ff5d327328bd68c2e86a4d9f2ba99bd0@chromium.org>
Commit-Queue: David Stevens <2f87ba95ff5d327328bd68c2e86a4d9f2ba99bd0@chromium.org>

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