Just a MAJOR warning that if you are running any servers with Xen-hypervisor. DO NOT UPGRADE to FC-22!!

There is a bug with GCC 5 that the package is compiled with and when installed will FAIL TO BOOT. This will result in any server you run Xen on to NOT BOOT.

This is confirmed in the source below:
* https://bugzilla.redhat.com/show_bug.cgi?id=1219197

If you had done it, there IS a way out!
Recovery
* Boot into non-Xen kernel (should work)
* Download the fc21 Xen basic packages
** qemu-img-2.1.2-6.fc21.x86_64.rpm
** xen-4.4.2-4.fc21.x86_64.rpm
** xen-hypervisor-4.4.2-4.fc21.x86_64.rpm
** xen-libs-4.4.2-4.fc21.x86_64.rpm
** xen-runtime-4.4.2-4.fc21.x86_64.rpm
* Uninstall all traces of new Xen (4.5.0-fc22)
yum erase xen xen-libs xen-hypervisor
* Install old packages back in
rpm -ivh *.rpm (assuming all the above downloads are the only RPMs in the current pwd)
* Reboot