sheepdog: check if '-o redundancy' is passed from user

This fix a segfault (that is caused by b3af018f3) of following command:

$ qemu-img convert some_img sheepdog:some_img

Cc: qemu-devel@nongnu.org
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
