it is bug in network-bridge, 
#^#
#vifnum=${vifnum:-$(ip route list | awk '/^default / { print $NF }' | sed 's/^[^0-9]*//')}
vifnum=${vifnum:-$(ip route list | sed -n -r -e "s/^default.+dev[^0-9]+([0-9]+).*$/\1/p")}
#$#

/etc/xen/scripts/network-bridge start netdev=eth0

Link veth100 is missing.
This may be because you have reached the limit of the number of interfaces
that the loopback driver supports.  If the loopback driver is a module, you
may raise this limit by passing it as a parameter (nloopbacks=<N>); if the
driver is compiled statically into the kernel, then you may set the parameter
using netloop.nloopbacks=<N> on the domain 0 kernel command line.