x86/pmu: Rename PC_VECTOR to PMI_VECTOR for better readability

The original name "PC_VECTOR" comes from the LVT Performance
Counter Register. Rename it to PMI_VECTOR. That's much more familiar
for KVM developers and it's still correct, e.g. it's the PMI vector
that's programmed into the LVT PC register.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Like Xu <likexu@tencent.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20221102225110.3023543-13-seanjc@google.com