extend -smp parsing to include cores= and threads= options

extend -smp parsing to include cores= and threads= options

For injecting multi-core and multi-threading CPU topology into guests
extend the -smp syntax to accommodate cores and threads specification.
Syntax: -smp smp_value[,cores=nr_cores][,threads=nr_threads]\
[,socket=nr_sockets][,maxcpus=max_cpus]
smp_value is the legacy value specifying the total number of vCPUs for
the guest. If you specify one of cores, threads or sockets this value
can be omitted. Missing values will be computed to fulfill:
smp_value = nr_cores * nr_threads * nr_sockets
where it will favour sockets over cores over threads (to mimic the
current behavior, which will only inject multiple sockets.)
So -smp 4,threads=2 will inject two sockets with 2 threads each,
-smp cores=4 is an abbreviation for -smp 4,cores=4,threads=1,sockets=1.
If max_cpus (the number of hotpluggable CPUs) is omitted, it will
be set to smp_value.

Signed-off-by: Andre Przywara <andre.przywara@amd.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>