wl: avoid inserting empty virtio queue entries
wl: avoid inserting empty virtio queue entries

The kernel driver currently short circuits the check for empty queue
entries if the entry arrives empty. Ordinarily the check is run every
time data is taken out of a queue entry and would recycle the entry once
empty. The short circuiting is being fixed in the kernel, but this
device change fixes the unnecessary empty queue entries from happening
in the first place.

BUG=chromium:791724
TEST=test code from the BUG

Change-Id: I5b72aac843def052bfe1234dfbde236274ae02bb
Reviewed-on: https://chromium-review.googlesource.com/974883
Commit-Ready: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>
Tested-by: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>
Reviewed-by: Stephen Barber <7719587e4b5a49340661b214ec85602f3918d06f@chromium.org>

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