parallels: error when no VM file found in source

The post-processor, when consuming a parallels artifact, checks for the
presence of a pvm or macvm file from the files of the artifact
received, and copies it in the output box for vagrant to use later.
However, if the artifact does not contain a compatible file, the
post-processor does not error, and instead returns a success, leading to
an unusable box being produced.

To avoid this, we count the files copied, and if none were, the box will
be unusable, so the post-processor errors now.

post-processor/vagrant/parallels.go

@@ -25,6 +25,8 @@ func (p *ParallelsProvider) Process(ui packersdk.Ui, artifact packersdk.Artifact
 	// Create the metadata
 	metadata = map[string]interface{}{"provider": "parallels"}
 
+	copied := 0
+
 	// Copy all of the original contents into the temporary directory
 	for _, path := range artifact.Files() {
 		// If the file isn't critical to the function of the
@@ -55,6 +57,11 @@ func (p *ParallelsProvider) Process(ui packersdk.Ui, artifact packersdk.Artifact
 		if err = CopyContents(dstPath, path); err != nil {
 			return
 		}
+		copied++
+	}
+
+	if copied == 0 {
+		err = fmt.Errorf("No VM file found in source artifact")
 	}
 
 	return