KVM: x86/vPMU: Implement AMD vPMU code for KVM

This patch replaces the empty AMD vPMU functions (in pmu_amd.c) with real
implementation.

Reviewed-by: Joerg Roedel <jroedel@suse.de>
Tested-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Wei Huang <wei@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>