./network-bridge start

Link veth1
0 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 loopback.nloopbacks=<N> on the domain 0 kernel command line.