plugin: only convert to negative errors on crosvm.h boundary
plugin: only convert to negative errors on crosvm.h boundary

We have decided that API defined in crosvm.h should signal errors by
returning negative values derived from common errno error codes. To
avoid confusion within the rest of crosvm code that is using positive
erro codes, let's perform the conversion to negative on that crosvm API
boundary, so it is contained.

TEST=cargo test --features plugin
BUG=None

Change-Id: Icb1e719c8e99c95fdc32dce13a30d6ff8d3d9dc7
Signed-off-by: Dmitry Torokhov <10a8c465cefc9bdd6c925e26964d23c90f1141cc@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/947563
Reviewed-by: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>

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