x86: vmx: Comprehensive max VMCS field search

Search all VMCS fields when looking for the max VMCS field index.
Perform the search backwards to save some time. Change the existing test
to compare the maximum expected index (based on MSRs) is equal to the
actual one.  This improves the test that currently performs
greater-equal comparison.

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