cros_async: Expose MemRegion to users
cros_async: Expose MemRegion to users

Allow users outside this crate to build MemRegions to user with
asynchronous IO.

Change-Id: Ia95f7c27cc5816181f61ca73641bec6fc11a0e3b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/2275723
Commit-Queue: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>
Tested-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>
Reviewed-by: Chirantan Ekbote <35787c0e27de5c48735e07f8324823b765e8bcbc@chromium.org>

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