drop lwip network support in stubdoms

drop lwip network support in stubdoms

in order to simplify network support in ioemu stubdoms, I am dropping
lwip based networking.
Network support is still provided the same way: using the tap
interface, created in qemu using netfront.
The lwip stack is still available to avoid additional compilation
issues.
However the stubdom is not going to have its own vif anymore,
this means that the only vnc server supported is the one in dom0.
You can still enable the vnc server in a stubdom at compile time, if
you want so.

Probably the most important change caused by this patch to xen users is
that you don't have to specify two vif in the stubdom config file
anymore, but just one:

-vif = [ '', 'ip=10.0.1.1,mac=aa:00:00:12:23:34']
+vif = ['ip=10.0.1.1,mac=aa:00:00:12:23:34']

this is the qemu part of the patch: we do not need to hard code 1 as the
starting vif for the tap interface anymore.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>