crosvm: avoid deprecated trim_left_matches
crosvm: avoid deprecated trim_left_matches

As of Rust 1.33.0, use of trim_left_matches warns that it is
deprecated and should be replaced with trim_start_matches.

BUG=None
TEST=build_test

Change-Id: I42bd5dba10aeffcd076882d28cb02551c50ff614
Signed-off-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1534963
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>

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