buildlet: add DestroyVM method to Client

And use it in gomote and release tools.

Change-Id: I87fa013d6d6729e7305dacd137be1b3d3b02f5f4
Reviewed-on: https://go-review.googlesource.com/3771
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
3 files changed