build-vm: Error out when initrd is not readable

Initrd of the system became only readable for root. So builds as user
fail when no kernel-obs-build is used.

We may should create an initrd. Maybe not on the fly, but via extra tooling.

build-vm-kvm

@@ -232,6 +232,9 @@ vm_verify_options_kvm() {
 	    fi
 	fi
     fi
+    if ! test -r "$vm_initrd" ; then
+	cleanup_and_exit 4 "Initrd file $vm_initrd is not readable."
+    fi
 
     case $kvm_device in
 	virtio*)