disk: add a fitTo() method to Partition that uses minSize()

Add a new Partition method, fitTo(), which resizes a partition to either
fit the given size or the sum of its contents + metadata.  The content
sum and metadata is calculated using the new minSize() method for
VolumeContainers.

pkg/disk/partition.go

@@ -59,6 +59,19 @@ func (p *Partition) GetChild(n uint) Entity {
 	return p.Payload
 }
 
+// fitTo resizes a partition to be either the given the size or the size of its
+// payload if that is larger. The payload can be larger if it is a container
+// with sized children.
+func (p *Partition) fitTo(size uint64) {
+	payload, isVC := p.Payload.(VolumeContainer)
+	if isVC {
+		if payloadMinSize := payload.minSize(size); payloadMinSize > size {
+			size = payloadMinSize
+		}
+	}
+	p.Size = size
+}
+
 func (p *Partition) GetSize() uint64 {
 	return p.Size
 }

pkg/disk/partition_table.go

@@ -423,6 +423,7 @@ func (pt *PartitionTable) relayout(size uint64) uint64 {
 			continue
 		}
 		partition.Start = start
+		partition.fitTo(partition.Size)
 		partition.Size = pt.AlignUp(partition.Size)
 		start += partition.Size
 	}
@@ -433,6 +434,7 @@ func (pt *PartitionTable) relayout(size uint64) uint64 {
 
 	root := &pt.Partitions[rootIdx]
 	root.Start = start
+	root.fitTo(root.Size)
 
 	// add the extra padding specified in the partition table
 	footer += pt.ExtraPadding