This commit enables partial operation of dummynet with kernels
compiled with "options VIMAGE".
As it is now, there is still a single instance of the pipes,
and it is only usable from vnet0 (the main instance).
Trying to use a pipe from a different vimage does not crash
the system as it did before, but the traffic coming out from
the pipe goes to the wrong place, and i still need to
figure out where.

Support for per-vimage pipes is almost there (just a matter of
uncommenting the VNET_* definitions for dn_cfg, plus putting into
the structure the remaining static variables), however i need
first to figure out how init/uninit work, and also to understand
where packets are ending up on exit from a pipe.

In summary: vimage support for dummynet is not complete yet,
but we are getting there.