vircapstest: Avoid (im)possible strcmp call with NULL argument

vircapstest: Avoid (im)possible strcmp call with NULL argument

Some compilers may get confused and decide we are calling strcmp with
NULL argument from test_virCapsDomainDataLookupLXC. Although this does
not really happen since the call is guarded with
(data->machinetype != expect_machinetype), using STRNEQ_NULLABLE is
easier to understand, less fragile, and doing so makes sure strcmp is
never called with NULL argument.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>