cros_async: Add Send to async_from
cros_async: Add Send to async_from

This allows struct using IoSourceExt to be Send

BUG=None
TEST=cargo test

Change-Id: I7541977151537387da59b0ada5592a77a0382998
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3002823
Reviewed-by: Chirantan Ekbote <35787c0e27de5c48735e07f8324823b765e8bcbc@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Commit-Queue: Woody Chow <cc0bff42e088f24a7e6e350b1e79cd0db9125d07@google.com>

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