storage: handle btrfs migration on different lxds
storage: handle btrfs migration on different lxds

If we receive btrfs subvolumes from an older LXD instance they are not called
".migration-send" but rather ".root". Handle this case.

Closes #3323.

Signed-off-by: Christian Brauner <48455ab3070520a2d174545c7239d6d0fabd9a83@ubuntu.com>

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