windows: fix build without --features=balloon
windows: fix build without --features=balloon

Add the minimal cfg checks to let the Windows target build without
--features=balloon enabled. This doesn't fully guard all uses of balloon
related types, just enough to make it compile, since it's intertwined
with other features (e.g. "kiwi").

BUG=None
TEST=cargo build --no-default-features

Change-Id: I013d84d07edb39d318bcd1be718a914e2282ea25
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/3885497
Reviewed-by: Noah Gold <2d4550d8af93bfc6e6fbe1fb1b0cc97a9ee7c3ce@google.com>
Commit-Queue: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>

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