If the kernel team accept backported patches then it might just be
possible to ship F15 with usable Domain 0 support but the timescale for
that would be very tight.
Talk to the kernel people. I think the chances to get the backend
drivers into the rawhide kernel even before the 2.6.38 merge window are
not that bad. Are they in linux-next already?
The situation is quite different from the 2.6.2x times when dom0 support
was dropped from the fedora kernels. The patches are pretty
self-contained now, i.e. they touch xen bits only instead of sprinkling
all over arch/x86/. Also the Xen people have reasonable and realistic
plan to get the stuff upstream soon.