qemuxml2xmltest: Introduce fake network driver

qemuxml2xmltest: Introduce fake network driver

In order to be able to use '<interface type="network"' we need a fake
network driver in qemuxml2argvtest. Create one by simply allowing users
to reuse configs from tests/networkxml2xmlin and tests/virnetworkportxml2xmldata
which will be returned to corresponding functions.

The driver implements:

    .networkLookupByName = fakeNetworkLookupByName,
      - validate syntax of network name, check if config exists
    .networkGetXMLDesc = fakeNetworkGetXMLDesc,
      - return appropriate XML
    .networkPortCreateXML = fakeNetworkPortCreateXML,
      - validate that port XML exists
    .networkPortGetXMLDesc = fakeNetworkPortGetXMLDesc,
      - return appropriate port XML

With the above and the correspondign test data, all network XMLs can be
enabled.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>