Mailing the story:
Set up Libvirt 0.7.7&Xen 4.0 on top of Fedora 13 RC2
Install Xen 4.0 and Libvirt like on F12 ( [1] ). Via my experience xend will hang for 5-7 min at startup and xen managed bridging will fail as well. At the same time PV guests won’t be able to obtain IP via interface virbr0. Workaround for the issue follows bellow.