devices: proxy: ensure device drop gets called
devices: proxy: ensure device drop gets called

Explicitly drop the device object before calling libc::exit() so that
the Drop impl for the device has a chance to be called.

Change-Id: Id93447925f71340c78a1d80efdff5b1574801409
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/3900327
Reviewed-by: Dennis Kempin <cd09796fb571bec2782819dbfd33307f65b1c778@google.com>
Commit-Queue: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>

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