vl: smp_parse: fix regression

Commit 0544edd8 "vl: smp_parse: cleanups" regressed any -smp
config that left either cores or threads unspecified, and specified
a topology supporting more cpus than the given online cpus. The
correct way to calculate the missing parameter would be to use
maxcpus, but it's too late to change that now. Restore the old
way, which is to calculate it with the online cpus (as is still
done), but then, if the result is zero, just set it to one.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Message-Id: <1466526844-29245-1-git-send-email-drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>