OvmfPkg/BaseMemEncryptTdxLib: Refactor error handle of SetOrClearSharedBit

OvmfPkg/BaseMemEncryptTdxLib: Refactor error handle of SetOrClearSharedBit

The previous implementation of SetOrClearSharedBit doesn't handle the
error correctly. In this patch SetOrClearSharedBit is changed to return
error code so that the caller can handle it.

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>
Cc: Michael Roth <michael.roth@amd.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
Signed-off-by: Min Xu <min.m.xu@intel.com>