vlib: add format_vnet_buffer_no_chain

 - add format_vnet_buffer and format_vnet_buffer_no_chain to mirror
   format_vlib_buffer and format_vlib_buffer_no_chain
 - format_vnet_buffer used to be the "no chain" version, replace all of
   its current use with the corresponding format_vnet_buffer_no_chain
 - add a function to dump vnet buffer details from gdb

Type: improvement

Change-Id: I143ce845f80e7ef937ea33a557b6e3b5988c5b8f
Signed-off-by: Benoît Ganne <bganne@cisco.com>