stubdom: drop lwip support

stubdom: drop lwip support

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']

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