boot.initrd.luks.devices: Change into an attribute set

This allows setting options for the same LUKS device in different
modules. For example, the auto-generated hardware-configuration.nix
can contain

  boot.initrd.luks.devices.crypted.device = "/dev/disk/...";

while configuration.nix can add

  boot.initrd.luks.devices.crypted.allowDiscards = true;

Also updated the examples/docs to use /disk/disk/by-uuid instead of
/dev/sda, since we shouldn't promote the use of the latter.

nixos/modules/virtualisation/qemu-vm.nix

@@ -465,7 +465,7 @@ in
       });
 
     swapDevices = mkVMOverride [ ];
-    boot.initrd.luks.devices = mkVMOverride [];
+    boot.initrd.luks.devices = mkVMOverride {};
 
     # Don't run ntpd in the guest.  It should get the correct time from KVM.
     services.ntp.enable = false;