vm_concierge: Add message to delete a vm disk

If a user wants to free up disk space or restart their VM from scratch
the existing disk needs to be deleted. Add a message to allow that.

BUG=815359
TEST=with other changes, vmc destroy <vm name>

Change-Id: I5131859c6a17d4332c9ecc8e97876029fd44e5aa
Reviewed-on: https://chromium-review.googlesource.com/935194
Commit-Ready: Dylan Reid <dgreid@chromium.org>
Tested-by: Dylan Reid <dgreid@chromium.org>
Reviewed-by: Stephen Barber <smbarber@chromium.org>
2 files changed