This patch is similar to 171e3d6b
which fixed qcow2:

Returning -EIO is far from optimal, but at least it's an error code.

Cc: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: NStefan Weil <weil@mail.berlios.de>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>