OvmfPkg: fix BuildResourceDescriptorHob call in PlatformAddHobCB()

BuildResourceDescriptorHob() expects the third parameter be the Length,
not the End address.

Fixes: 328076cf ("OvmfPkg/PlatformInitLib: Add PlatformAddHobCB")
Reported-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Tested-by: Tom Lendacky <thomas.lendacky@amd.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>