qemuxmlconftest: Separate fake drivers into a separate file

qemuxmlconftest: Separate fake drivers into a separate file

One of the tests that qemuxmlconftest does is generate cmd line
for given domain XML. This process might involve talking to other
drivers (secret/storage/nwfilter/network). To produce predictable
output the test comes with fake implementation of APIs of those
drivers. Well, move that implementation into a separate file so
that it can be reused by other tests (notably, qemuhotplugtest is
going to use it).

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Laine Stump <laine@redhat.com>