x86: vmx: Fix the check whether CMCI is supported

The logic of figuring out whether CMCI is supported is broken, causing
the CMCI accessing tests to fail on Skylake bare-metal.

Determine whether CMCI is supported according to the maximum entries in
the LVT as encoded in the APIC version register.

Suggested-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Nadav Amit <namit@vmware.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>