tools/osbuild-mpp: add sector size support for image layouts

Now you can specify a sector_size in `mpp-define-images` to support
creating a 4k native disk image (sector_size=4096).

This does use a loopback device, which means osbuild-mpp also needs
to run as root, when previously that wasn't necessary.

tools/osbuild-mpp

@@ -974,13 +974,19 @@ class Image:
     def from_dict(cls, js):
         size = js["size"]
         data = js["table"]
+        sector_size = js.get('sector_size', 512)
 
         with tempfile.TemporaryDirectory() as tmp:
             image = os.path.join(tmp, "disk.img")
             subprocess.run(["truncate", "--size", size, image], check=True)
-
-            table = PartitionTable.from_dict(data)
-            table.write_to(image)
+            cp = subprocess.run(["losetup", "--find", "--show", f"--sector-size={sector_size}", image],
+                                stdout=subprocess.PIPE, check=True)
+            loopimage = cp.stdout.rstrip()
+            try:
+                table = PartitionTable.from_dict(data)
+                table.write_to(loopimage)
+            finally:
+                subprocess.run(["losetup", "-d", loopimage])
 
         return cls(size, table)