set CPUID bits to present cores and threads topology

Controlled by the enhanced -smp option set the CPUID bits to present the
guest the desired topology. This is vendor specific, but (with the exception
of the CMP_LEGACY bit) not conflicting, so we set all bits everytime.
There is no real multithreading support for AMD CPUs, so report cores
instead.

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