Prevent partial VM startup failures from leaking loop devices and dm state on the host.
Move root snapshot setup into a rollback-safe helper that records loop and mapper handles incrementally, tears them down in reverse order on failure, and reuses the same dm/loop cleanup path during normal runtime teardown. Also switch the daemon runner field to a small command-runner interface so the snapshot path can be tested with injected failures.
Add failure-injection coverage for losetup, blockdev, dmsetup, partial teardown, and joined rollback errors. Validated with go test ./... and make build.
