types: fix negative relative disk size

example/negative-size.nix

@@ -0,0 +1,27 @@
+{ disks ? [ "/dev/vdb" "/dev/vdc" ], ... }: {
+  disk = {
+    disk0 = {
+      device = builtins.elemAt disks 0;
+      type = "disk";
+      content = {
+        type = "table";
+        format = "gpt";
+        partitions = [
+          {
+            name = "nix";
+            type = "partition";
+            part-type = "primary";
+            start = "0%";
+            end = "-10MiB";
+            bootable = true;
+            content = {
+              type = "filesystem";
+              format = "ext4";
+              mountpoint = "/";
+            };
+          }
+        ];
+      };
+    };
+  };
+}