Ok, so 2 differences here:
~/models/FVP_Base_Cortex-A57-A53x14/models/Linux64_GCC-4.1/FVP_Base_Cortex-A57x4-A53x4
You are using a 64bit build of the model, I use a 32bit build from day to day. I found there are problems with the virtioblock device when run on 32bit models... maybe this is another case... I will check.
bp.virtioblockdevice.image_path=sd.img
What is the sd.img you are passing in? (hopefully that isn't the rootfs)
Mark