linux: drop VM before exiting to allow cleanup
linux: drop VM before exiting to allow cleanup

Clean up the `linux` object (which contains the devices) before the
control sockets passed to `run_control` are closed.  This allows crosvm
to shut down cleanly without any error messages about short reads from
the control sockets.

BUG=chromium:992494
TEST=exit crosvm without errors

Change-Id: I1040c2f9ecbd03f820c7082da3327962ecc445f1
Signed-off-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/1802155
Reviewed-by: Stephen Barber <7719587e4b5a49340661b214ec85602f3918d06f@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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