parasite: Report negative value from parasite_execure

The value in eax is positive on error. Callers expect it to
be negative to report error.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

parasite-syscall.c

@@ -246,7 +246,7 @@ static int parasite_execute_by_pid(unsigned long cmd, struct parasite_ctl *ctl,
 
 	ret = __parasite_execute(ctl, pid, &regs);
 	if (ret == 0)
-		ret = (int)regs.ax;
+		ret = -(int)regs.ax;
 
 	if (args)
 		memcpy(args, ctl->addr_args, args_size);