raw-posix: fix O_DIRECT short reads

The following O_DIRECT read from a <512 byte file fails:

  $ truncate -s 320 test.img
  $ qemu-io -n -c 'read -P 0 0 512' test.img
  qemu-io: can't open device test.img: Could not read image for determining its format: Invalid argument

Note that qemu-io completes successfully without the -n (O_DIRECT)
option.

This patch fixes qemu-iotests ./check -nocache -vmdk 059.

Cc: qemu-stable@nongnu.org
Suggested-by: Kevin Wolf <kwolf@redhat.com>
Reported-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
