qemu-nbd: properly report error if qemu_daemon() is failed

errno has been overwritten by dup2() just below qemu_daemon() and thus
improperly returned to the caller. Fix accordingly.

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Eric Blake <eblake@redhat.com>
CC: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Message-ID: <20230717145544.194786-5-den@openvz.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: reorder patch series]
Signed-off-by: Eric Blake <eblake@redhat.com>

qemu-nbd.c

@@ -932,14 +932,17 @@ int main(int argc, char **argv)
             error_report("Failed to fork: %s", strerror(errno));
             exit(EXIT_FAILURE);
         } else if (pid == 0) {
+            int saved_errno;
+
             close(stderr_fd[0]);
 
             ret = qemu_daemon(1, 0);
+            saved_errno = errno;    /* dup2 will overwrite error below */
 
             /* Temporarily redirect stderr to the parent's pipe...  */
             dup2(stderr_fd[1], STDERR_FILENO);
             if (ret < 0) {
-                error_report("Failed to daemonize: %s", strerror(errno));
+                error_report("Failed to daemonize: %s", strerror(saved_errno));
                 exit(EXIT_FAILURE);
             }