QEMU might support more values for "-drive discard", so using Bi-state
values (on/off) for it doesn't make sense.
"on" maps to "unmap", "off" maps to "ignore":
<...>
@var{discard} is one of "ignore" (or "off") or "unmap" (or "on") and
controls whether @dfn{discard} (also known as @dfn{trim} or @dfn{unmap})
requests are ignored or passed to the filesystem. Some machine types
may not support discard requests.
</...>
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