Merge pull request #2629 from mitchellh/b-2601

Document Qemu file extension for vm_name

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)
 	}

website/source/docs/builders/qemu.html.markdown

@@ -247,8 +247,10 @@ builder and not otherwise conflicting with the qemuargs):
     host port.
 
 -   `vm_name` (string) - This is the name of the image (QCOW2 or IMG) file for
-    the new virtual machine, without the file extension. By default this is
+    the new virtual machine. By default this is "packer-BUILDNAME.FORMAT", where
-    "packer-BUILDNAME", where "BUILDNAME" is the name of the build.
+    `BUILDNAME` is the name of the build and `FORMAT` matches the `format`
+    option, above. If you customize this you must specify the file extension or
+    none will be used.
 
 -   `vnc_port_min` and `vnc_port_max` (integer) - The minimum and maximum port
     to use for the VNC port on the host machine which is forwarded to the VNC