KVM: x86/pmu: preserve IA32_PERF_CAPABILITIES across CPUID refresh

Once MSR_IA32_PERF_CAPABILITIES is changed via vmx_set_msr(), the
value should not be changed by cpuid(). To ensure that the new value
is kept, the default initialization path is moved to intel_pmu_init().
The effective value of the MSR will be 0 if PDCM is clear, however.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>