Network Bridge in dom0 can not ping domU's
You need to connect dom0 to the xenbr1 bridge. Think of a bridge as an ethernet switch, you need to "plug" dom0 into it to work. You will notice that dom0 is connected to xenbr0 via eth0.
If you have an additional interface on the host(eth1), you can simply do:
brctl addif xenbr1 eth1
This should work provided the IP networking is properly configured.