driver: `ListSnapshotsResponse` should return []struct, not a machine-unreadable string

[AkihiroSuda]

lima/pkg/driver/external/driver.proto

Lines 207 to 211 in de0816e

	// ListSnapshotsResponse carries the existing snapshots.
	message ListSnapshotsResponse {
	// Driver-formatted list of existing snapshots.
	string snapshots = 1;
	}

It should return a slice of struct, not a raw output of qemu-img snapshot -l which is not robustly parsable.

Depends on:

• Support the equivalent of qemu-img snapshot --list command go-qcow2reader#75

[afbjorklund]

When you are done with this API change, make sure to change pkg/snapshot and cmd/limactl/snapshot too