port.lua selftest(): Transmit on all intel10g ports at once.

Very hack-and-slash. Have to fix the "transmits descriptor ring
doesn't loop" bug before this will be interesting.

Here is how the output looks now:

$ make && sudo flock -x /tmp/snabb.lock ./snabbswitch -e 'require("port").selftest()' | grep TXDGPC
make: Nothing to be done for `all'.
              TXDGPC           32,767 DMA Good Tx Packet Counter
              TXDGPC           32,767 DMA Good Tx Packet Counter
              TXDGPC           32,767 DMA Good Tx Packet Counter
              TXDGPC           32,767 DMA Good Tx Packet Counter
              TXDGPC           32,767 DMA Good Tx Packet Counter
              TXDGPC           32,767 DMA Good Tx Packet Counter
              TXDGPC           32,767 DMA Good Tx Packet Counter
              TXDGPC           32,767 DMA Good Tx Packet Counter
              TXDGPC           32,767 DMA Good Tx Packet Counter
              TXDGPC           32,767 DMA Good Tx Packet Counter
              TXDGPC           32,767 DMA Good Tx Packet Counter
              TXDGPC           32,767 DMA Good Tx Packet Counter
              TXDGPC           32,767 DMA Good Tx Packet Counter
              TXDGPC           32,767 DMA Good Tx Packet Counter
              TXDGPC           32,767 DMA Good Tx Packet Counter
              TXDGPC           32,767 DMA Good Tx Packet Counter
              TXDGPC           32,767 DMA Good Tx Packet Counter
              TXDGPC           32,767 DMA Good Tx Packet Counter
              TXDGPC           32,767 DMA Good Tx Packet Counter
              TXDGPC           32,767 DMA Good Tx Packet Counter