disk: honour the fact that some containers have metadata

Re-introduce the VolumeContainer interface but with a different
meaning: it is supposed to be implemented by all container that
contain volumes and as a result have themselves a size, like eg
LVM2, LUKS2 and PartitionTable (the latter is not yet included).
The sole method on the interface for now is MetadataSize, which
should return the metadata for the container itself.
Use that new `VolumeContainer.MetadataSize` method when we up-
date the sizes of elements in `resizeEntitybranch`.

internal/disk/lvm.go

@@ -105,6 +105,18 @@ func (vg *LVMVolumeGroup) CreateMountpoint(mountpoint string, size uint64) (Enti
 	return &vg.LogicalVolumes[len(vg.LogicalVolumes)-1], nil
 }
 
+func (vg *LVMVolumeGroup) MetadataSize() uint64 {
+	if vg == nil {
+		return 0
+	}
+
+	// LVM2 allows for a lot of customizations that will affect the size
+	// of the metadata and its location and thus the start of the physical
+	// extent. For now we assume the default which results in a start of
+	// the physical extent 1 MiB
+	return 1024 * 1024
+}
+
 type LVMLogicalVolume struct {
 	Name    string
 	Size    uint64