vl: smp_parse: cleanups

No functional changes; only some code movement and removal of
dead code (impossible conditions). Also, max_cpus can be
initialized to 1, like smp_cpus, because it's either set by the
user or set to smp_cpus, when smp_cpus is set by the user, or
set to 1, when nothing is set.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Message-Id: <1465580427-13596-2-git-send-email-drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>