raw-posix: Handle errors in raw_create

In qemu-iotests, some large images are created using qemu-img.

Without checks for errors, qemu-img will just create an
empty image, and later read / write tests will fail.

With the patch, failures during image creation are detected
and reported.

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>