By default, Qemu will now have an extension added based on the file format. This is a change to more closely match behavior from 0.7.5.

builder/qemu/builder.go

@@ -201,14 +201,15 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 		b.config.VNCPortMax = 6000
 	}
 
-	if b.config.VMName == "" {
-		b.config.VMName = fmt.Sprintf("packer-%s", b.config.PackerBuildName)
-	}
-
 	if b.config.Format == "" {
 		b.config.Format = "qcow2"
 	}
 
+	if b.config.VMName == "" {
+		b.config.VMName = fmt.Sprintf("packer-%s.%s",
+			b.config.PackerBuildName, b.config.Format)
+	}
+
 	if b.config.FloppyFiles == nil {
 		b.config.FloppyFiles = make([]string, 0)
 	}