dom0+vm/vif-script: setup IP address of net backend interface
dom0+vm/vif-script: setup IP address of net backend interface

This is needed to connect to ProxyVM/NetVM, not only pass traffic ahead. Still
firewall rules applies.

QubesOS/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,QubesOS/qubes-core-admin,woju/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin