x86: apic: Track APIC ops on a per-cpu basis

Track the virtual function table to handle xAPIC vs. x2APIC on a per-cpu
basis.  Using a common global is racy as nothing in KUT synchronizes
CPUs when switching to/from x2APIC.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220121231852.1439917-8-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>