vmm_vhost: Add TODO comments about IoSlice::advance_slice()
vmm_vhost: Add TODO comments about IoSlice::advance_slice()

BUG=b:194137301
TEST=cargo test

Change-Id: I28466597209e4e81dc566d66d69875f59808eb0f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3317325
Reviewed-by: Chirantan Ekbote <35787c0e27de5c48735e07f8324823b765e8bcbc@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Commit-Queue: Keiichi Watanabe <815e4e8f7c92a23424d7c3f51ead622bcedbc720@chromium.org>

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