Add support for AMD Core Performance counters in the guest. The base
event select and counter MSRs are changed. In addition, with the core
extension, there are 2 extra counters available for performance
measurements for a total of 6.

With the new MSRs, the logic to map them to the gp_counters[] is changed.
New functions are added to check the validity of the get/set MSRs.

If the guest has the X86_FEATURE_PERFCTR_CORE cpuid flag set, the number
of counters available to the vcpu is set to 6. It the flag is not set
then it is 4.
Signed-off-by: Janakarajan Natarajan <Janakarajan.Natarajan@amd.com>
[Squashed "Expose AMD Core Perf Extension flag to guests" - Radim.]
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>