data_model: Export IoSliceMut as a public type
data_model: Export IoSliceMut as a public type

This type is useful in other places as well.

BUG=none
TEST=unit tests

Change-Id: I38a8db6e82809121ef8ce336ca0e3da80625d60a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/2571148
Reviewed-by: Noah Gold <2d4550d8af93bfc6e6fbe1fb1b0cc97a9ee7c3ce@google.com>
Reviewed-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Commit-Queue: Chirantan Ekbote <35787c0e27de5c48735e07f8324823b765e8bcbc@chromium.org>

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