d/rules: use disable-download option instead of git-submodules=ignore

d/rules: use disable-download option instead of git-submodules=ignore

See the following QEMU commits for reference:
0c5f3dcbb2 ("configure: add --enable-pypi and --disable-pypi")
ac4ccac740 ("configure: rename --enable-pypi to --enable-download, control subprojects too")
6f3ae23b29 ("configure: remove --with-git-submodules=") removed

The last one removed the option and the closest thing to
git-submodule=ignore is using disable-download. Which will then just
verify that the submodules are present.

Building now will require running either
* Running 'meson subprojects download' in the qemu submodule first.
* Using --enable-download, but then the submodules would be downloaded
  for each build (if not already downloaded in the submodule first)
  and it's just a bit too surprising if downloads happen during build.

The disable-download option will also disable automatic downloading of
missing Python modules from PyPI. Hopefully, it's enough to add them
as Debian build dependencies when required.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>