KVM: TDX: Enable guest access to MTRR MSRs

Allow TDX guests to access MTRR MSRs as what KVM does for normal VMs, i.e.,
KVM emulates accesses to MTRR MSRs, but doesn't virtualize guest MTRR
memory types.

TDX module exposes MTRR feature to TDX guests unconditionally.  KVM needs
to support MTRR MSRs accesses for TDX guests to match the architectural
behavior.

Signed-off-by: Binbin Wu <binbin.wu@linux.intel.com>
Message-ID: <20250227012021.1778144-19-binbin.wu@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>