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.
</...>
QEMU introduced "discard" option for drive since commit a9384aff53,
<...>
@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.
</...>
This patch exposes the support in libvirt.
QEMU supported "discard" for "-drive" since v1.5.0-rc0:
% git tag --contains a9384aff53
contains
v1.5.0-rc0
v1.5.0-rc1
So this only detects the capability bit using virQEMUCapsProbeQMPCommandLine.
