KVM: x86: Reject userspace attempts to access PERF_CAPABILITIES w/o PDCM

KVM: x86: Reject userspace attempts to access PERF_CAPABILITIES w/o PDCM

Reject userspace accesses to PERF_CAPABILITIES if PDCM isn't set in guest
CPUID, i.e. if the vCPU doesn't actually have PERF_CAPABILITIES.  But!  Do
so via KVM_MSR_RET_UNSUPPORTED, so that reads get '0' and writes of '0'
are ignored if KVM advertised support PERF_CAPABILITIES.

KVM's ABI is that userspace must set guest CPUID prior to setting MSRs,
and that setting MSRs that aren't supposed exist is disallowed (modulo the
'0' exemption).

Link: https://lore.kernel.org/r/20240802185511.305849-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>