do_guest_openat /proc interposition doesn't work for openat
Description of problem:
For instance, trying with hppa emulated on top of x86:

```
$ hppa-linux-gnu-gcc test.c -o test
$ qemu-hppa-static ./test
```

One gets the host cpu information:

```
processor	: 0
vendor_id	: GenuineIntel
cpu family	: 6
model		: 142
model name	: Intel(R) Core(TM) i5-10210U CPU @ 1.60GHz
[...]
```

while we would want to see the guest cpu information, like the test program does when `#if 0` is turned into `#if 1`:

```
processor	: 0            
cpu family	: PA-RISC 1.1e
cpu		: PA7300LC (PCX-L2)
capabilities	: os32
model		: 9000/778/B160L - Merlin L2 160 QEMU (9000/778/B160L)
```

This is because `do_guest_openat` only checks for the path, and does not look at `dirfd`, so it doesn't recognize that `openat(dirfd, "cpuinfo", O_RDONLY)` is actually opening a file in `/proc`.

We could probably, when `dirfd` is not `AT_FDCWD`, try to `fstat()` it, open `/proc` with `O_DIRECTORY` and `fstat()` that too, and compare their `st_dev` and `st_ino`?