Clarification
What the warning means is that the patch is targeted *only* for deployment within a Xen system; and 1) probably will not be acceptable as-is in the core qemu project ("may not be appropriate for adoption upstream"), and 2) may not actually fix the problem if you're using QEMU outside of a normal Xen system -- for instance, in KVM, or in your own virtualization system ("...or in other contexts").
In other words, this patch is a hack -- a safe and effective one for Xen, but not a long-term solution. Normally we would always try to provide a proper fix, but in this case a proper fix would require changes to the interface, which can't be done effectively in a security update.
The text you quote is a bit unclear; it's difficult sometimes to get every detail just right when you're under time pressure and focusing on trying to get everything else right.