storage/reflink: add revision even if empty
storage/reflink: add revision even if empty

It's sort of useful to be able to revert a volume that has only ever
been started once to its empty state. And the lvm_thin driver allows it
too, so why not.

marmarek/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,QubesOS/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin