ixgbevf: Add code to check for Tx hang

This patch adds code to allow for Tx hang checking.  The idea is to provide
more robust debug info in the event of a transmit unit hang. Similar to the
logic in ixgbe.

CC: Alexander Duyck <alexander.h.duyck@redhat.com>
Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>