add crosvm user and group

BUG=none
TEST=install crosvm ebuild and see crosvm user

Change-Id: I0620dd73461aeb5f55fa9d8941f023deb45ddddc
Reviewed-on: https://chromium-review.googlesource.com/733859
Commit-Ready: Stephen Barber <smbarber@chromium.org>
Tested-by: Stephen Barber <smbarber@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>