Add a crate containing an API for controlling the balloon. This is done
by reusing the API between the control socket and the balloon device.
BUG=b:213962590
TEST=compiles
Change-Id: I5d89d5ebee55162d28ba45ca2c49b7083561b916
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3394440
Reviewed-by: Keiichi Watanabe <keiichiw@chromium.org>
Tested-by: kokoro <noreply+kokoro@google.com>
Commit-Queue: David Stevens <stevensd@chromium.org>
