OvmfPkg/Microvm: add PcdConfidentialComputingGuestAttr

OvmfPkg/Microvm: add PcdConfidentialComputingGuestAttr

Fixes build failure:

/home/kraxel/projects/edk2/OvmfPkg/PlatformPei/AmdSev.c: In function ‘AmdSevInitialize’:
/home/kraxel/projects/edk2/MdePkg/Include/Library/PcdLib.h:511:38: error: implicit declaration of function ‘_PCD_SET_MODE_64_S_PcdConfidentialComputingGuestAttr’ [-Werror=implicit-function-declaration]
  511 | #define PcdSet64S(TokenName, Value)  _PCD_SET_MODE_64_S_##TokenName    ((Value))
      |                                      ^~~~~~~~~~~~~~~~~~~
/home/kraxel/projects/edk2/OvmfPkg/PlatformPei/AmdSev.c:410:17: note: in expansion of macro ‘PcdSet64S’
  410 |     PcdStatus = PcdSet64S (PcdConfidentialComputingGuestAttr, CCAttrAmdSevSnp);
      |                 ^~~~~~~~~
cc1: all warnings being treated as errors

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>