qemuxml2argvtest: Adapt to ethernet automatic tap creation

qemuxml2argvtest: Adapt to ethernet automatic tap creation

After 9c17d665fdc5 the tap device for ethernet network type is
automatically precreated before spawning qemu. Problem is, the
qemuxml2argvtest wasn't updated and thus is failing. Because of
all the APIs that new code is calling, I had to mock a lot. Also,
since the tap FDs are labeled separately from the rest of the
devices/files I had to enable NOP security driver for the test
too.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>