KVM: x86/pmu: Register PMI handler for mediated vPMU

Register a dedicated PMI handler with perf's callback when mediated PMU
support is enabled.  Perf routes PMIs that arrive while guest context is
loaded to the provided callback, by modifying the CPU's LVTPC to point at
a dedicated mediated PMI IRQ vector.

WARN upon receipt of a mediated PMI if there is no active vCPU, or if the
vCPU doesn't have a mediated PMU.  Even if a PMI manages to skid past
VM-Exit, it should never be delayed all the way beyond unloading the vCPU.
And while running vCPUs without a mediated PMU, the LVTPC should never be
wired up to the mediated PMI IRQ vector, i.e. should always be routed
through perf's NMI handler.

Signed-off-by: Xiong Zhang <xiong.y.zhang@linux.intel.com>
Signed-off-by: Mingwei Zhang <mizhang@google.com>
Tested-by: Xudong Hao <xudong.hao@intel.com>
Co-developed-by: Sean Christopherson <seanjc@google.com>
Tested-by: Manali Shukla <manali.shukla@amd.com>
Link: https://patch.msgid.link/20251206001720.468579-20-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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