vl.c: Add -smp, dies=* command line support and update doc

vl.c: Add -smp, dies=* command line support and update doc

For PC target, users could configure the number of dies per one package
via command line with this patch, such as "-smp dies=2,cores=4".

The parsing rules of new cpu-topology model obey the same restrictions/logic
as the legacy socket/core/thread model especially on missing values computing.

Signed-off-by: Like Xu <like.xu@linux.intel.com>
Message-Id: <20190620054525.37188-4-like.xu@linux.intel.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>