Pre-fill user buffers on passthrough commands

Pre-fill user buffers on passthrough commands

Reported that user data may be shorter than the buffer length, and need
to pad the data with a fixed byte value.

Also requested that we provied a way to prefill data in buffers so we
can know if a device is corrupting memory.

Signed-off-by: Keith Busch <keith.busch@intel.com>