Rollback partial dm snapshot startup

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.

internal/daemon/snapshot.go

@@ -0,0 +1,79 @@
+package daemon
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"strings"
+)
+
+type dmSnapshotHandles struct {
+	BaseLoop string
+	COWLoop  string
+	DMName   string
+	DMDev    string
+}
+
+func (d *Daemon) createDMSnapshot(ctx context.Context, rootfsPath, cowPath, dmName string) (handles dmSnapshotHandles, err error) {
+	defer func() {
+		if err == nil {
+			return
+		}
+		if cleanupErr := d.cleanupDMSnapshot(context.Background(), handles); cleanupErr != nil {
+			err = errors.Join(err, cleanupErr)
+		}
+	}()
+
+	baseBytes, err := d.runner.RunSudo(ctx, "losetup", "-f", "--show", "--read-only", rootfsPath)
+	if err != nil {
+		return handles, err
+	}
+	handles.BaseLoop = strings.TrimSpace(string(baseBytes))
+
+	cowBytes, err := d.runner.RunSudo(ctx, "losetup", "-f", "--show", cowPath)
+	if err != nil {
+		return handles, err
+	}
+	handles.COWLoop = strings.TrimSpace(string(cowBytes))
+
+	sectorsBytes, err := d.runner.RunSudo(ctx, "blockdev", "--getsz", handles.BaseLoop)
+	if err != nil {
+		return handles, err
+	}
+	sectors := strings.TrimSpace(string(sectorsBytes))
+
+	if _, err := d.runner.RunSudo(ctx, "dmsetup", "create", dmName, "--table", fmt.Sprintf("0 %s snapshot %s %s P 8", sectors, handles.BaseLoop, handles.COWLoop)); err != nil {
+		return handles, err
+	}
+	handles.DMName = dmName
+	handles.DMDev = "/dev/mapper/" + dmName
+	return handles, nil
+}
+
+func (d *Daemon) cleanupDMSnapshot(ctx context.Context, handles dmSnapshotHandles) error {
+	var cleanupErr error
+
+	switch {
+	case handles.DMName != "":
+		if _, err := d.runner.RunSudo(ctx, "dmsetup", "remove", handles.DMName); err != nil {
+			cleanupErr = errors.Join(cleanupErr, err)
+		}
+	case handles.DMDev != "":
+		if _, err := d.runner.RunSudo(ctx, "dmsetup", "remove", handles.DMDev); err != nil {
+			cleanupErr = errors.Join(cleanupErr, err)
+		}
+	}
+
+	if handles.COWLoop != "" {
+		if _, err := d.runner.RunSudo(ctx, "losetup", "-d", handles.COWLoop); err != nil {
+			cleanupErr = errors.Join(cleanupErr, err)
+		}
+	}
+	if handles.BaseLoop != "" {
+		if _, err := d.runner.RunSudo(ctx, "losetup", "-d", handles.BaseLoop); err != nil {
+			cleanupErr = errors.Join(cleanupErr, err)
+		}
+	}
+
+	return cleanupErr
+}