fix vhd format support

closes  #47

Signed-off-by: Adphi <[email protected]>

builder.go

@@ -44,7 +44,7 @@ ff02::3 ip6-allhosts
 	perm os.FileMode = 0644
 )
 
-var formats = []string{"qcow2", "qed", "raw", "vdi", "vhd", "vmdk"}
+var formats = []string{"qcow2", "qed", "raw", "vdi", "vhd", "vhd", "vhdx", "vmdk"}
 
 type Builder interface {
 	Build(ctx context.Context) (err error)
@@ -113,6 +113,9 @@ func NewBuilder(ctx context.Context, workdir, imgTag, disk string, size uint64,
 	if !valid {
 		return nil, fmt.Errorf("invalid format: %s valid formats are: %s", f, strings.Join(formats, " "))
 	}
+	if f == "vhd" {
+		f = "vpc"
+	}
 
 	if splitBoot && bootSize < 50 {
 		return nil, fmt.Errorf("boot partition size must be at least 50MiB")