MdePkg: Add CC_GUEST_TYPE in ConfidentialComputingGuestAttr.h

MdePkg: Add CC_GUEST_TYPE in ConfidentialComputingGuestAttr.h

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3902

The confidential computing guest type (GUEST_TYPE) was defined in
OvmfPkg/Include/WorkArea.h. Now it is to be moved to
MdePkg/Include/ConfidentialComputingGuestAttr.h and renamed as
CC_GUEST_TYPE.

There are 2 reasons for this change.
1. CC_GUEST_TYPE is a generic definition and will be used in CcProbeLib
   which is defined in MdePkg.
2. Based on the latest edk2 coding style:
 - First character should be upper case
 - Must contain lower case characters
 - No white space characters
 - Global variable name must start with a 'g'

As the first step CC_GUEST_TYPE is defined in this patch. In the
next patch GUEST_TYPE will be deleted. This is to make sure the
bisect work correctly.

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Zhiguang Liu <zhiguang.liu@intel.com>
Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: Erdem Aktas <erdemaktas@google.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Min Xu <min.m.xu@intel.com>