vmxnet3: coverity woe

vmxnet3: coverity woe

Coverity complains that hb might be null when !prev_b0 is true. It may be right, but not likely.
To make the code more explicit when it parses a complete packet, set got_packet = 1 and test
got_packet instead of prev_b0 which is somewhat obscure.

Use vlib_buffer_free_one rather than vlib_buffer_free_no_next in output.c which takes care of
chain buffers.

Change-Id: I6e0587605fdc622c3f55fedd74c474666631ed91
Signed-off-by: Steven <sluong@cisco.com>