meson: fix Windows build

The build fails on Windows. Replace calls to Unix programs like ´cat´,
´sed´ and ´true´ with calls to ´python´ and wrap calls to
´os.path.relpath´ in try-except because it can fail when the two paths
are on different drives. Make sure to convert the Windows paths to Unix
paths to prevent warnings in generated files.

Signed-off-by: oltolm <oleg.tolmatcev@gmail.com>
Message-id: 20250612221521.1109-2-oleg.tolmatcev@gmail.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

tests/include/meson.build

@@ -13,4 +13,4 @@ test_qapi_outputs_extra = [
 test_qapi_files_extra = custom_target('QAPI test (include)',
                                       output: test_qapi_outputs_extra,
                                       input: test_qapi_files,
-                                      command: 'true')
+                                      command: [python, '-c', ''])