KVM: x86: Use guest MAXPHYADDR from CPUID.0x8000_0008 iff TDP is enabled

commit 4bf48e3c0aafd32b960d341c4925b48f416f14a5 upstream.

Ignore the guest MAXPHYADDR reported by CPUID.0x8000_0008 if TDP, i.e.
NPT, is disabled, and instead use the host's MAXPHYADDR.  Per AMD'S APM:

  Maximum guest physical address size in bits. This number applies only
  to guests using nested paging. When this field is zero, refer to the
  PhysAddrSize field for the maximum guest physical address size.

Fixes: 24c82e57 ("KVM: Sanitize cpuid")
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210623230552.4027702-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>