osbuild2: new stages and osbuild features

Stages:
- org.osbuild.copy
- org.osbuild.truncate
- org.osbuild.sfdisk
- org.osbuild.qemu
- org.osbuild.mkfs.btrfs
- org.osbuild.mkfs.ext4
- org.osbuild.mkfs.fat
- org.osbuild.mkfs.xfs
- org.osbuild.grub2.inst

Stages can now have devices and mounts in addition to options and
inputs.

Devices:
- org.osbuild.loopback

Mounts:
- org.osbuild.btrfs
- org.osbuild.ext4
- org.osbuild.fat
- org.osbuild.xfs

internal/osbuild2/loopback_device.go

@@ -0,0 +1,26 @@
+package osbuild2
+
+// Expose a file (or part of it) as a device node
+
+type LoopbackDeviceOptions struct {
+	// File to associate with the loopback device
+	Filename string `json:"filename"`
+
+	// Start of the data segment
+	Start uint64 `json:"start,omitempty"`
+
+	// Size limit of the data segment (in sectors)
+	Size uint64 `json:"size,omitempty"`
+
+	// Sector size (in bytes)
+	SectorSize *uint64 `json:"sector-size,omitempty"`
+}
+
+func (LoopbackDeviceOptions) isDeviceOptions() {}
+
+func NewLoopbackDevice(options *LoopbackDeviceOptions) *Device {
+	return &Device{
+		Type:    "org.osbuild.loopback",
+		Options: options,
+	}
+}