btrfs: beef-up btrfs support with a limited understanding of quota

With this change we understand more than just leaf quota groups for
btrfs file systems. Specifically:

- When we create a subvolume we can now optionally add the new subvolume
  to all qgroups its parent subvolume was member of too. Alternatively
  it is also possible to insert an intermediary quota group between the
  parent's qgroups and the subvolume's leaf qgroup, which is useful for
  a concept of "subtree" qgroups, that contain a subvolume and all its
  children.

- The remove logic for subvolumes has been updated to optionally remove
  any leaf qgroups or "subtree" qgroups, following the logic above.

- The snapshot logic for subvolumes has been updated to replicate the
  original qgroup setup of the source, if it follows the "subtree"
  design described above. It will not cover qgroup setups that introduce
  arbitrary qgroups, especially those orthogonal to the subvolume
  hierarchy.

This also tries to be more graceful when setting up /var/lib/machines as
btrfs. For example, if mkfs.btrfs is missing we don't even try to set it
up as loopback device.

Fixes #1559
Fixes #1129

src/basic/missing.h

@@ -494,6 +494,10 @@ struct btrfs_ioctl_quota_ctl_args {
 #define BTRFS_QGROUP_LIMIT_KEY 244
 #endif
 
+#ifndef BTRFS_QGROUP_RELATION_KEY
+#define BTRFS_QGROUP_RELATION_KEY 246
+#endif
+
 #ifndef BTRFS_ROOT_BACKREF_KEY
 #define BTRFS_ROOT_BACKREF_KEY 144
 #endif