I doubt someone will comment this until unembargoed.
After unembargoed:
a. Many Xen-related issues are not applicable for Qubes, at least not directly, because QEMU is not considered as a part of TCB. (Although QEMU seems to begin considered as additional line of defense in 4.0.)
b. If it is a critical issue, there will be a QSB for it.
Moreover, we still don't know the severity.
Regards,
Vít Šesták 'v6ak'