kvm tools: Enable O_DIRECT support

kvm tools: Enable O_DIRECT support

With Direct I/O, file reads and writes go directly from the applications
to the storage device, bypassing the operating system read and write
caches. This is useful for applications that manage their own caches.

Open a disk image with O_DIRECT:
   $ lkvm run -d ~/img/test.img,direct

The original readonly flag is still supported.
Open a disk image with O_DIRECT and readonly:
   $ lkvm run -d ~/img/test.img,direct,ro

Signed-off-by: Asias He <asias.hejun@gmail.com>
Acked-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>