vl: Don't silently change topology when all -smp options were set

QEMU tries to change the "threads" option even if it was explicitly set
in the command-line, and it shouldn't do that.

The right thing to do when all options (cpus, sockets, cores, threds)
are explicitly set is to sanity check them and abort in case they don't
make sense (i.e. when sockets*cores*threads < cpus).

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
