coverage: update the score

After updating the CI the coverage value changed causing failures:

    Current code coverage (76.33%) deviates by 7.67% from the
    previous code coverage 84.00%.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>

coverage_config_x86_64.json

@@ -1,5 +1,5 @@
 {
-  "coverage_score": 84.0,
+  "coverage_score": 76.3,
   "exclude_path": "vhost/src/vhost_kern/",
   "crate_features": "vhost/vhost-user-frontend,vhost/vhost-user-backend"
 }