OvmfPkg/QemuFwCfgLib: rewrite fw_cfg probe

Move the code to a new QemuFwCfgProbe() function.  Use direct Io*() calls
instead of indirect QemuFwCfg*() calls to make sure we don't get
recursive calls.  Also simplify CC guest detection.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Tested-by: Tom Lendacky <thomas.lendacky@amd.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>

OvmfPkg/Library/QemuFwCfgLib/QemuFwCfgPeiLib.inf

@@ -41,7 +41,6 @@
   DebugLib
   IoLib
   MemoryAllocationLib
-  MemEncryptSevLib
 
 [Pcd]
   gUefiOvmfPkgTokenSpaceGuid.PcdOvmfWorkAreaBase