If desired netvm presence is different than during savefile creation(*),
defer setting the netvm until new DispVM is running - otherwise kernel
there will not notice the change and will either have (not working)
'eth0' when it shouldn't, or will not have it while it should.
Additionally set dispvm.uses_default_netvm = False, so GUI tools will
display actual netvm value.
(*) Actually compare to netvm set for dispvm template (`TEMPLATE-dvm`
VM), which can be different if user just changed that but not
regenerated dispvm savefile yet.
Fixesqubesos/qubes-issues#985
Related to qubesos/qubes-issues#862
77da23fba2