nssmock: Prefer free() over VIR_FREE()

nssmock: Prefer free() over VIR_FREE()

Problem with VIR_FREE() is that we are not linking
libvirt-utils.so to our mock libs therefore there will be an
unresolved symbol. Fortunately, nsstest that eventually links
with the nssmock links also with libvirt-utils.so and thus the
symbol is resolved after all. However, if one wants to run the
test binary under valgrind it is impossible to do so. Because of
the unresolved symbol.

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