qemuBuildNumaArgStr: Discard def->cpu check

qemuBuildNumaArgStr: Discard def->cpu check

In the function at one place we check if def->cpu is NULL prior
to accessing def->cpu->ncells. Then, later in the code,
def->cpu->ncells is accessed directly, without the check. This
makes coverity unhappy, because the first check makes it think
def->cpu can be NULL. However, the function is not called if
def->cpu is NULL. Therefore, remove the first check and hopefully
make coverity cheer again.

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