Revert "KVM: x86/pmu: Accept 0 for absent PMU MSRs when host-initiated if !enable_pmu"

Revert "KVM: x86/pmu: Accept 0 for absent PMU MSRs when host-initiated if !enable_pmu"

Eating reads and writes to all "PMU" MSRs when there is no PMU is wildly
broken as it results in allowing accesses to _any_ MSR on Intel CPUs
as intel_is_valid_msr() returns true for all host_initiated accesses.

A revert of commit 217ed595e7c4 ("KVM: x86: always allow host-initiated
writes to PMU MSRs") will soon follow.

This reverts commit e6a4f3f1441f0babfc5cd7dc8e6872ab576a4fe6.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220611005755.753273-4-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>