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.

This is similiar with physical world, one will be surprised if the
box starts with medium exists while the tray is open.

New tests are added, tests disk-{cdrom,floppy}-tray are for the qemu
supports "-device" flag, and disk-{cdrom,floppy}-no-device-cap are
for old qemu, i.e. which doesn't support "-device" flag.