syscall_emul: Return correct writev value

According to Linux man pages, if writev is successful, it returns the total
number of bytes written. Otherwise, it returns an error code. Instead of
returning 0, return the result from the actual call to writev in the system
call.

src/sim/syscall_emul.hh

@@ -1138,7 +1138,7 @@ writevFunc(SyscallDesc *desc, int callnum, LiveProcess *process,
     if (result < 0)
         return -errno;
 
-    return 0;
+    return result;
 }