disk: resolve iter_nth_zero clippy warning
disk: resolve iter_nth_zero clippy warning

Replace .nth(0) with .next() to resolve this:

  error: called `.nth(0)` on a `std::iter::Iterator`

BUG=None
TEST=bin/clippy

Change-Id: I103e8b44b1564708d92d096aaf044db7f67d1ba8
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/2151147
Reviewed-by: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>
Tested-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Commit-Queue: Keiichi Watanabe <815e4e8f7c92a23424d7c3f51ead622bcedbc720@chromium.org>

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