ci: add xen to features for coverage

Now that all features are additive, collect coverage with the xen
feature enabled.

Signed-off-by: Patrick Roy <[email protected]>

Author: roypat

coverage_config_x86_64.json

@@ -1,5 +1,5 @@
 {
   "coverage_score": 91.78,
   "exclude_path": "mmap_windows.rs",
-  "crate_features": "backend-mmap,backend-atomic,backend-bitmap"
+  "crate_features": "backend-mmap,backend-atomic,backend-bitmap,xen"
 }