data_model: allow reading structs from io::Read
data_model: allow reading structs from io::Read

Add a from_reader() function that allows reading a DataInit type from a
std::io::Read stream.

BUG=None
TEST=./build_test

Change-Id: Iaacae489db2c265d6b1a9af6e18c11c9e6efe354
Signed-off-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/1737723
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>

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