[virtio-9p] Ignore O_DIRECT hint from client.

The O_DIRECT flag imposes alignment restrictions on the length and address
of userspace buffers and the file offset of I/Os.

While VirtFS/9P has plans to implement O_DIRECT behavior on the server,
for now we will stick to a behavior like NFS by bypassing the page cache
only on the client. Server may still cache the I/O.

Signed-off-by: Venkateswararao Jujjuri <jvrao@linux.vnet.ibm.com>
