net_util: use sys_util::Error instead of std::io::Error
net_util: use sys_util::Error instead of std::io::Error

sys_util errors always have a backing errno. Use these to allow
clients to always get access to the underlying errno when using
ioctl-based operations.

BUG=none
TEST=cargo test

Change-Id: I8853eeb2b9b38e5509c0ef01f507e461d5d36267
Reviewed-on: https://chromium-review.googlesource.com/927804
Commit-Ready: Stephen Barber <7719587e4b5a49340661b214ec85602f3918d06f@chromium.org>
Tested-by: Stephen Barber <7719587e4b5a49340661b214ec85602f3918d06f@chromium.org>
Reviewed-by: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>
Reviewed-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>

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