crosvm: fix queue wrapping behavior over u16
crosvm: fix queue wrapping behavior over u16

The queue's next index, as set by the kernel, will increase
monotonically to the limit of a u16 and then wrap back to zero. In
contrast, Rust will panic when its internal counter reaches the limit.
This change enables the wrapping behavior that we want.

TEST=run a kernel that uses a queue >0xffff times
BUG=None

Change-Id: I24101dfb137a1350fd93d1b378840b72431764ea
Reviewed-on: https://chromium-review.googlesource.com/598502
Commit-Ready: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>
Tested-by: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>
Reviewed-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>

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