I think I had the same problem.
I had this in /var/log/xen/qemu-dm-u1010.log :
domid: 3
-c config qemu network with xen bridge for
tap3.0 xenbr0
bridge xenbr0 does not exist!
/etc/xen/scripts/qemu-ifup: could not launch network script
Could not initialize device ‘tap’
To resolve, just add xenbr0 bridge, and link it on tap0 (or other). This is normaly doing by /etc/xen/scripts/qemu-ifup, by in my case bridge did not exists…
brctl addbr xenbr0
ifconfig tap0 0.0.0.0 up
brctl addif xenbr0 tap0