osbuild2: add new GenImage{Prepare,Finish}Stages

Add new helper functions that will return the necessary stages to
prepare a disk image, i.e. take care of creating the raw image
file, partitioning it, creating the devices on it (LUKS2, LVM2),
and formatting all the file systems: `GenImageStagesPrepare`.
Additionally, some partition layouts require some post-processing,
e.g. LVM2 where the volume group needs to be renamed "offline".
For this the `GenImageFinishStages` was added.

Co-Authored-By: Achilleas Koutsou <achilleas@koutsou.net>

internal/osbuild2/disk.go

@@ -0,0 +1,65 @@
+package osbuild2
+
+import (
+	"fmt"
+
+	"github.com/osbuild/osbuild-composer/internal/disk"
+)
+
+// sfdiskStageOptions creates the options and devices properties for an
+// org.osbuild.sfdisk stage based on a partition table description
+func sfdiskStageOptions(pt *disk.PartitionTable) *SfdiskStageOptions {
+	partitions := make([]Partition, len(pt.Partitions))
+	for idx, p := range pt.Partitions {
+		partitions[idx] = Partition{
+			Bootable: p.Bootable,
+			Start:    pt.BytesToSectors(p.Start),
+			Size:     pt.BytesToSectors(p.Size),
+			Type:     p.Type,
+			UUID:     p.UUID,
+		}
+	}
+	stageOptions := &SfdiskStageOptions{
+		Label:      pt.Type,
+		UUID:       pt.UUID,
+		Partitions: partitions,
+	}
+
+	return stageOptions
+}
+
+func GenImagePrepareStages(pt *disk.PartitionTable, filename string) []*Stage {
+	stages := make([]*Stage, 0)
+
+	// create an empty file of the given size via `org.osbuild.truncate`
+	stage := NewTruncateStage(
+		&TruncateStageOptions{
+			Filename: filename,
+			Size:     fmt.Sprintf("%d", pt.Size),
+		})
+
+	stages = append(stages, stage)
+
+	// create the partition layout in the empty file
+	sfOptions := sfdiskStageOptions(pt)
+	loopback := NewLoopbackDevice(
+		&LoopbackDeviceOptions{Filename: filename},
+	)
+
+	sfdisk := NewSfdiskStage(sfOptions, loopback)
+	stages = append(stages, sfdisk)
+
+	// Generate all the needed "devices", like LUKS2 and LVM2
+	s := GenDeviceCreationStages(pt, filename)
+	stages = append(stages, s...)
+
+	// Generate all the filesystems on partitons and devices
+	s = GenMkfsStages(pt, loopback)
+	stages = append(stages, s...)
+
+	return stages
+}
+
+func GenImageFinishStages(pt *disk.PartitionTable, filename string) []*Stage {
+	return GenDeviceFinishStages(pt, filename)
+}