common: log loopback detaches

We want to find the culprit loopback device which isn't detatching on
crbug.com/1141907. We might as well log our cleanup actions anyway, and
this will allow us to see the last loopback processed in production.

BUG=chromium:1141907
TEST=just a log message
BRANCH=None
Signed-off-by: George Engelbrecht <engeg@google.com>
Change-Id: I126efceae4f67993069675c23f6c4af61c7e5667
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/2514561
Reviewed-by: LaMont Jones <lamontjones@chromium.org>