Yikes. https://www.qubes-os.org/news/2017/04/04/qsb-29/
Xen is looking more and more as a liability. Subgraph OS which takes a completely different approach (sandboxing + hardened kernel) has a much better track record with the only vulnerability being dirtycow. Qubes has been affected multiple times due to Xen bugs in recent years: https://www.qubes-os.org/security/xsa/