KVM: x86/pmu: Disallow emulation in the fastpath if mediated PMCs are active

KVM: x86/pmu: Disallow emulation in the fastpath if mediated PMCs are active

Don't handle exits in the fastpath if emulation is required, i.e. if an
instruction needs to be skipped, the mediated PMU is enabled, and one or
more PMCs is counting instructions.  With the mediated PMU, KVM's cache of
PMU state is inconsistent with respect to hardware until KVM exits the
inner run loop (when the mediated PMU is "put").

Reviewed-by: Sandipan Das <sandipan.das@amd.com>
Tested-by: Xudong Hao <xudong.hao@intel.com>
Tested-by: Manali Shukla <manali.shukla@amd.com>
Link: https://patch.msgid.link/20251206001720.468579-29-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>