manifest: support specifying force_size for VPC

The RHEL 7 vpc subformat in qemu does not support force_size so we need
to be able to disable it.  The parameter in all parts is defined as a
pointer because the default value is 'true'.  Not specifying it will
keep the option in the osbuild stage as 'nil', falling back to 'true' in
osbuild.

internal/image/live.go

@@ -25,6 +25,7 @@ type LiveImage struct {
 	Workload         workload.Workload
 	Filename         string
 	Compression      string
+	ForceSize        *bool
 	PartTool         osbuild.PartTool
 
 	NoBLS     bool
@@ -82,6 +83,7 @@ func (img *LiveImage) InstantiateManifest(m *manifest.Manifest,
 		if img.Compression == "" {
 			vpcPipeline.Filename = img.Filename
 		}
+		vpcPipeline.ForceSize = img.ForceSize
 		artifactPipeline = vpcPipeline
 		artifact = vpcPipeline.Export()
 	case platform.FORMAT_VMDK: