KVM: x86: Add helpers to handle 64-bit APIC MSR read/writes

Add helpers to handle 64-bit APIC read/writes via MSRs to deduplicate the
x2APIC and Hyper-V code needed to service reads/writes to ICR.  Future
support for IPI virtualization will add yet another path where KVM must
handle 64-bit APIC MSR reads/write (to ICR).

Opportunistically fix the comment in the write path; ICR2 holds the
destination (if there's no shorthand), not the vector.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220204214205.3306634-9-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>