vl: Add another sanity check to smp_parse() function
The code in smp_parse already checks the topology information for
sockets * cores * threads < cpus and bails out with an error in
that case. However, it is still possible to supply a bad configuration
the other way round, e.g. with:
QEMU then still starts the guest, with topology configuration that
is rather incomprehensible and likely not what the user wanted.
So let's add another check to refuse such wrong configurations.