intel10g: Fix old (mostly harmless) bug in RX descriptor creation.

Store zero instead of length following the pointer. (It's not a length
field.)
intel10g optimization: no register reads for TX/RX.

Detect DMA progress by inspecting memory instead of TDH/RDH registers.
Reading registers with MMIO is too expensive even for the sync step.
(Writes are OK.)
Merge pull request #197 from lukego/optimize-loadgen

Optimize intel10g for loadgen benchmark