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/luks.go

@@ -74,3 +74,12 @@ func (lc *LUKSContainer) GenUUID(rng *rand.Rand) {
 		lc.UUID = uuid.Must(newRandomUUIDFromReader(rng)).String()
 	}
 }
+
+func (lc *LUKSContainer) MetadataSize() uint64 {
+	if lc == nil {
+		return 0
+	}
+
+	// 16 MiB is the default size for the LUKS2 header
+	return 16 * 1024 * 1024
+}