virtio_blk: fix endianess annotations

Since commit 72e61eb4 (virtio: change config
to guest endian) config space is no longer fixed endian.

Lets change the virtio_blk_config variables.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>