qemu-x86_64 don't support unshare(CLONE_NEWUSER)
Description of problem:
Why qemu-x86_64 call unshare(CLONE_NEWUSER) fail?
```
    fuzzing@ubuntu:~/Desktop/afl/AFLplusplus$ qemu-x86_64 /bin/unshare --user /bin/bash
    unshare: unshare failed: Invalid argument
    fuzzing@ubuntu:~/Desktop/afl/AFLplusplus$ /bin/unshare --user /bin/bash
    nobody@ubuntu:~/Desktop/afl/AFLplusplus$
```
Steps to reproduce:
1.execute `qemu-x86_64 /bin/unshare --user /bin/bash` ,it will fail <br/>
2.execute `/bin/unshare --user /bin/bash` ,it will ok

How i fix that?