distro/rhel9: define qcow2 image type in function

The global qcow2ImgType was unconfigured and meant to be used by calling
the mkQcow2ImgType() function.  In the distro initialisation code, the
new variable created from the mkQcow2ImgType() function with the
configuration was shadowing the global variable.  This can lead to
errors in the future where it's not clear which variable is used where.

Putting the definition in a function makes it impossible to use without
configuring.

internal/distro/rhel9/qcow2.go

@@ -8,23 +8,6 @@ import (
 )
 
 var (
-	qcow2ImgType = imageType{
-		name:          "qcow2",
-		filename:      "disk.qcow2",
-		mimeType:      "application/x-qemu-disk",
-		kernelOptions: "console=tty0 console=ttyS0,115200n8 no_timer_check net.ifnames=0",
-		packageSets: map[string]packageSetFunc{
-			osPkgsKey: qcow2CommonPackageSet,
-		},
-		bootable:            true,
-		defaultSize:         10 * common.GibiByte,
-		image:               liveImage,
-		buildPipelines:      []string{"build"},
-		payloadPipelines:    []string{"os", "image", "qcow2"},
-		exports:             []string{"qcow2"},
-		basePartitionTables: defaultBasePartitionTables,
-	}
-
 	openstackImgType = imageType{
 		name:     "openstack",
 		filename: "disk.qcow2",
@@ -167,7 +150,22 @@ func qcowImageConfig(d distribution) *distro.ImageConfig {
 }
 
 func mkQcow2ImgType(d distribution) imageType {
-	it := qcow2ImgType
+	it := imageType{
+		name:          "qcow2",
+		filename:      "disk.qcow2",
+		mimeType:      "application/x-qemu-disk",
+		kernelOptions: "console=tty0 console=ttyS0,115200n8 no_timer_check net.ifnames=0",
+		packageSets: map[string]packageSetFunc{
+			osPkgsKey: qcow2CommonPackageSet,
+		},
+		bootable:            true,
+		defaultSize:         10 * common.GibiByte,
+		image:               liveImage,
+		buildPipelines:      []string{"build"},
+		payloadPipelines:    []string{"os", "image", "qcow2"},
+		exports:             []string{"qcow2"},
+		basePartitionTables: defaultBasePartitionTables,
+	}
 	it.defaultImageConfig = qcowImageConfig(d)
 	return it
 }