OvmfPkg: fix PlatformConfig
The Hii form is named "MainFormState" and the EFI variable is named
"PlatformConfig". Take into account the different names.
Fixes: aefcc91805fd ("OvmfPkg/PlatformDxe: Handle all requests in ExtractConfig and RouteConfig")
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>