This question is asked in regards of post http://wiki.xenproject.org/wiki/Xen_via_libvirt_for_OpenStack_juno
One of stopper issues seems to be  commit :-
http://git.qemu.org/?p=qemu.git&a=search&h=2001e197cf68a4f262a5a06e0ffe6dce3c2e2a7b&st=commit&s=xen_disk%3A+fix+unmapping+of+persistent+grants

So, the question is mostly related with RDO Openstack release on F22 having qemu version 2.3.0-rc1.
Another concern is requirement to install package "nova-compute-xen" on compute node.

Thanks.
Boris.