Comment 4 for bug 1913012

Gentoo also noticed the bug: https://bugs.gentoo.org/766743

Jannik Glückert proposed a fix:

```
--- a/pc-bios/descriptors/meson.build
+++ b/pc-bios/descriptors/meson.build
@@ -8,7 +8,7 @@ foreach f: [
 ]
   configure_file(input: files(f),
                  output: f,
- configuration: {'DATADIR': qemu_datadir},
+ configuration: {'DATADIR': get_option('prefix') / qemu_datadir},
                  install: get_option('install_blobs'),
                  install_dir: qemu_datadir / 'firmware')
 endforeach
```