add example to add empty disks to a VM

Author: Mic92

examples/empty-disks.nix

@@ -0,0 +1,5 @@
+{
+  # This is useful to test installations and partitions.
+  # Those will show up as /dev/vdb and /dev/vdc.
+  virtualisation.emptyDiskImages = [ 4096 4096 ];
+}