avocado_qemu: explicitly return None to avoid R1710

The linter is complaining the `pick_default_qemu_bin` is not explicitly
returning None. Fix it to explicitly return None and avoid R1710
inconsistent-return-statements.

Signed-off-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210920204932.94132-6-willianr@redhat.com>

tests/acceptance/avocado_qemu/__init__.py

@@ -72,6 +72,7 @@ def pick_default_qemu_bin(arch=None):
                                               qemu_bin_relative_path)
     if is_readable_executable_file(qemu_bin_from_bld_dir_path):
         return qemu_bin_from_bld_dir_path
+    return None
 
 
 def _console_interaction(test, success_message, failure_message,