Virtualized SINIC fixes

Virtualized SINIC fixes

dev/pktfifo.hh:
    we can't modify i because it's used further down to remove
    the packet from the fifo.  Instead, copy the iterator and
    modify that to get the previous packet.
dev/sinic.cc:
    - don't change the transmit state and kick the machine unless
    we're at the head of the txList.
    - add a couple of debugging statements to figure out how far
    along we've gotten in processing a packet.
    - assert that the current tx vnic has something to do when
    we start processing the state machine.

--HG--
extra : convert_revision : 588fe2c7d810be0e3d8d39c5cc0ec8a72119517e