qemu/iovec: Don't assert if sbytes is zero

Since these values can possibly be sent from guest (for hw/9pfs), do a sanity check
on them. A 9p write request with 0 bytes caused qemu to abort without this patch

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
