target-ppc: Fix number of threads per core limit

The number of threads per core is different for POWER6/7/8 CPUs.
Guest systems do not expect to see more threads per core than
a specific CPU supports so we need to limit this number.
This limit is implemented by ppc_get_compat_smt_threads().

However it has a problem as it checks for PCR (Processor Compatibility
Register) mask, 2.05 means 2 threads per core, 2.06 - 4 threads.
For POWER8 one would expect PCR_COMPAT_2_07 bit set and
ppc_get_compat_smt_threads() checking for it to return 8 threads
per core. But the latest PowerISA spec now is 2.07 and there is
no 2.07 compatibility mode defined, QEMU does not define it either
(will be in PowerISA 2.08).

Instead of relying on a PCR mask, this uses kvmppc_smt_threads()
which returns the maximum supported threads number for KVM or
1 for TCG.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Alexander Graf <agraf@suse.de>