btrfs: Change default subvol upon creation

The default root subvol comes with its own limitations and it might be
better off changing the default subvol upon creation. This should also
let us create hidden subvols that may be used for storing snapshots,
without exposing them to the end-user.

declarative.py

@@ -1,5 +1,6 @@
 import os
 import subprocess
+import tempfile
 from pathlib import Path
 
 from util import run
@@ -70,6 +71,19 @@ def be_formatted(dev, fs):
             run(f"mkfs.ext4 -m 0 {device}")
         elif fs == "btrfs":
             run(f"mkfs.btrfs {device}")
+            tmp_mnt = tempfile.mkdtemp(prefix="mnt-")
+            default_subvol = f"{tmp_mnt}/default"
+            run(
+                f"""
+                set -ex
+                mkdir -p {tmp_mnt}
+                mount {device} {tmp_mnt}
+                btrfs subvolume create {default_subvol}
+                btrfs subvolume set-default {default_subvol}
+                umount {tmp_mnt}
+                rmdir {tmp_mnt}
+                """
+            )
         else:
             raise Exception(f"Unsupported fs type: {fs}")