When fail, translate_desc() returns negative value, otherwise the
number of iovs. So we should fail when the return value is negative
instead of a blindly check against zero.

Detected by CoverityScan, CID# 1442593:  Control flow issues  (DEADCODE)

Fixes: cc5e7107 ("vhost: log dirty page correctly")
Acked-by: NMichael S. Tsirkin <mst@redhat.com>
Reported-by: NStephen Hemminger <stephen@networkplumber.org>
Signed-off-by: NJason Wang <jasowang@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>