Fixed the dropped packet issue, head and tail were reversed in rx_pending().
Added Intel 82571 support to Snabb Switch

Here are the key changes required:

 * PCI device ID: I had to change so a suitable device would be found.

 * PHY reads/writes: PHY reads and writes are synchronized by using the
   PHY locking functions.

 * PHY reset procedure: The procedure to reset the 82571 card is
   vastly different than the 82574 due to the synchronization issues
   associated with the multi-port card.

 * PHY status differences: The 82571 PHY status register varies
   slightly from the 82574. For example, the copper link status is not
   available in bit 3 like it is on the 82574. This resulted in a
   couple of changes in the way stats are printed.

 * PHY locking code: The 82571 uses different registers to obtain a
   PHY lock from firmware. In addition, I had to implement software
   locking (SWSM.SMBI) to synchronize access between drivers. Recall
   that this is a multi-port card, so it is possible that one port
   could be bound to snabbswitch and the other bound to the OS. The
   locking methods take an options parameter that allows one to
   request the type of lock desired (software, firmware, or both). By
   default, both locks are acquired and released. However, during the
   PHY reset process, there were steps that had to be executed in
   between the acquisition of software and firmware locks, which is
   why I refactored the code in this manner.