disk: fix compile-time warning
disk: fix compile-time warning

Do not warn if the `image_path` parameter is unused, as it is only used
if the composite-disk feature is enabled.

BUG=b:221887571
TEST=cargo build

Change-Id: I91f747152c0e5f4d75f7522a33d35f91890c6df6
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3531731
Reviewed-by: Keiichi Watanabe <815e4e8f7c92a23424d7c3f51ead622bcedbc720@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Commit-Queue: Alexandre Courbot <301cd6a3ea50a894b4b51e385c5eb12b9a4ffc0c@chromium.org>

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