Allocate virtualisation.diskSize with useBootLoader

nixos/modules/virtualisation/qemu-vm.nix

@@ -136,14 +136,20 @@ let
             ''
               # Create a writable qcow2 image using the systemImage as a backing
               # image.
+              BACKING_SIZE_MB=$(( $(${lib.getExe' qemu "qemu-img"} info ${systemImage}/nixos.qcow2 --output=json | ${lib.getExe hostPkgs.jq} -r '."virtual-size"') / 1024 / 1024 ))
+              DISK_SIZE_MB=${toString cfg.diskSize}
+              if (( DISK_SIZE_MB < BACKING_SIZE_MB )); then
+                OVERLAY_SIZE_MB=$BACKING_SIZE_MB
+              else
+                OVERLAY_SIZE_MB=$DISK_SIZE_MB
+              fi
 
-              # CoW prevent size to be attributed to an image.
-              # FIXME: raise this issue to upstream.
               ${qemu}/bin/qemu-img create \
                 -f qcow2 \
                 -b ${systemImage}/nixos.qcow2 \
                 -F qcow2 \
-                "$NIX_DISK_IMAGE"
+                "$NIX_DISK_IMAGE" \
+                 ''${OVERLAY_SIZE_MB}M
             ''
           else if cfg.useDefaultFilesystems then
             ''