qemu-io: remove unnecessary assignment

Remove an unnecessary assignment, spotted by clang analyzer:
/src/qemu/qemu-io.c:995:9: warning: Value stored to 'offset' is never read
        offset += reqs[i].qiov->size;

Acked-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

qemu-io.c

@@ -992,7 +992,6 @@ static int multiwrite_f(int argc, char **argv)
 
         optind = j + 1;
 
-        offset += reqs[i].qiov->size;
         pattern++;
     }