lxd/storage: Fix custom volume copies
lxd/storage: Fix custom volume copies

On ZFS, when a pool name doesn't match the zpool name or dataset, the
wrong paths were being accessed.

Closes #5339

Signed-off-by: Stéphane Graber <089afc6d81f66f1168a9849e15660feae286e024@ubuntu.com>

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