Currently, we use a broad assumption that pv domains == service vms ==
available network backends. With an hvm ndvm, this assumption is broken.

As a simple solution, add a field called 'is_network_backend' to the
top-level Domain type definition. This will just be set manually, but
will allow clients like the GUI to have a better way of filtering
available network backends.

Finally, update the CreateNDVM function to set this field to true, and
check for this field in the redctl GUI when making a list of available
backends.

Signed-off-by: Nick Rosbrook <rosbrookn@ainfosec.com>