runtime: Allow and require no initrd for SE

Previously, it was not permitted to have neither an initrd nor an image.
However, this is the exact config to use for Secure Execution, where the
initrd is part of the image to be specified as `-kernel`. Require the
configuration of no initrd for Secure Execution.

Also
- remove redundant code for image/initrd checking -- no need to check in
  `newQemuHypervisorConfig` (calling) when it is also checked in
  `getInitrdAndImage` (called)
- use `QemuCCWVirtio` constant when possible

Fixes: #3922
Signed-off-by: Jakob Naucke <jakob.naucke@ibm.com>

src/runtime/virtcontainers/qemu.go

@@ -1841,7 +1841,7 @@ func (q *qemu) hotplugAddCPUs(amount uint32) (uint32, error) {
 		threadID := fmt.Sprintf("%d", hc.Properties.Thread)
 
 		// If CPU type is IBM pSeries, Z or arm virt, we do not set socketID and threadID
-		if machine.Type == "pseries" || machine.Type == "s390-ccw-virtio" || machine.Type == "virt" {
+		if machine.Type == "pseries" || machine.Type == QemuCCWVirtio || machine.Type == "virt" {
 			socketID = ""
 			threadID = ""
 			dieID = ""