8.1.0rc0: configure from tar file fetches subprojects via git
Description of problem:
Executing configure from tar file fetches subprojects via git. Fetched subprojects are https://gitlab.com/qemu-project/dtc and https://gitlab.com/qemu-project/keycodemapdb
```
$ ./configure --disable-download
Using './build' as the directory for build output
...
Initialized empty Git repository in /home/helge/qemu-8.1.0-rc0/subprojects/dtc/.git/
remote: Enumerating objects: 319, done.
remote: Counting objects: 100% (319/319), done.
remote: Compressing objects: 100% (251/251), done.
remote: Total 319 (delta 54), reused 163 (delta 38), pack-reused 0
Receiving objects: 100% (319/319), 250.56 KiB | 1.94 MiB/s, done.
Resolving deltas: 100% (54/54), done.
From https://gitlab.com/qemu-project/dtc
 * branch            b6910bec11614980a21e46fbccc35934b671bd81 -> FETCH_HEAD
HEAD is now at b6910be Bump version to v1.6.1
...
Initialized empty Git repository in /home/helge/qemu-8.1.0-rc0/subprojects/keycodemapdb/.git/
remote: Enumerating objects: 26, done.
remote: Counting objects: 100% (26/26), done.
remote: Compressing objects: 100% (21/21), done.
remote: Total 26 (delta 0), reused 23 (delta 0), pack-reused 0
Unpacking objects: 100% (26/26), 30.65 KiB | 216.00 KiB/s, done.
From https://gitlab.com/qemu-project/keycodemapdb
 * branch            f5772a62ec52591ff6870b7e8ef32482371f22c6 -> FETCH_HEAD
HEAD is now at f5772a6 Add Qemu qcode support for F13 to F24
...
```

Using `--disable-download` is no option:
```
$ ./configure --disable-download
Using './build' as the directory for build output

ERROR: missing subprojects

This is not a GIT checkout but subproject content appears to
be missing. Do not use 'git archive' or GitHub download links
to acquire QEMU source archives. Non-GIT builds are only
supported with source archives linked from:

  https://www.qemu.org/download/#source
```

If I understand the error message correctly, the subprojects should be part of the tar.
Steps to reproduce:
1. Open Clang64 console
2. `pacman -Syu`
3. `pacman -S binutils mingw-w64-clang-x86_64-toolchain mingw-w64-clang-x86_64-glib2 mingw-w64-clang-x86_64-ninja mingw-w64-clang-x86_64-pixman mingw-w64-clang-x86_64-python mingw-w64-clang-x86_64-python-sphinx mingw-w64-clang-x86_64-python-sphinx_rtd_theme`
4. `wget https://download.qemu.org/qemu-8.1.0-rc0.tar.xz`
5. `tar -xf qemu-8.1.0-rc0.tar.xz`
6. `cd qemu-8.1.0-rc0`
7. `./configure` or `./configure --disable-download`