crosvm: clean up waiting for children
crosvm: clean up waiting for children

Signed-off-by: Stephen Barber <7719587e4b5a49340661b214ec85602f3918d06f@chromium.org>

BUG=none
TEST=run and kill block device process

Change-Id: I1a4e98cb1985bfeb2303428f95f3bae27dccf803
Reviewed-on: https://chromium-review.googlesource.com/576463
Commit-Ready: Stephen Barber <7719587e4b5a49340661b214ec85602f3918d06f@chromium.org>
Tested-by: Stephen Barber <7719587e4b5a49340661b214ec85602f3918d06f@chromium.org>
Reviewed-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>

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