OvmfPkg/AcpiPlatformDxe: Check PcdConfidentialComputingGuestAttr

OvmfPkg/AcpiPlatformDxe: Check PcdConfidentialComputingGuestAttr

PcdConfidentialComputingGuestAttr can be used to check the cc guest
type, including td-guest or sev-guest. CcProbe() can do the same
thing but CcProbeLib should be included in the dsc which uses
AcpiPlatformDxe. The difference between PcdConfidentialComputingGuestAttr
and CcProbe() is that PcdConfidentialComputingGuestAttr cannot be used
in multi-processor scenario but CcProbe() can. But there is no such
issue in AcpiPlatformDxe.

So we use PcdConfidentialComputingGuestAttr instead of CcProbeLib so that
it is simpler.

Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Erdem Aktas <erdemaktas@google.com>
Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Min Xu <min.m.xu@intel.com>