KVM: x86/vPMU: reset pmc->counter to 0 for pmu fixed_counters

To avoid semantic inconsistency, the fixed_counters in Intel vPMU
need to be reset to 0 in intel_pmu_reset() as gp_counters does.

Signed-off-by: Like Xu <like.xu@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>