qemu: Support discard for disk

qemu: Support discard for disk

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.