avf: check duplicate pci address upon create interface

avf: check duplicate pci address upon create interface

Entering duplicate pci address when creating an avf interface causes
crash in register_node

(gdb) f 4
145	    error_exit (1);
(gdb) up
354	      clib_error ("more than one node named `%v'", n->name);
(gdb) p n
$1 = (vlib_node_t *) 0x7fffbbe55de4
(gdb) p n->name
$2 = (u8 *) 0x7fffbc410b10 "avf-0/4/a/0-tx"
(gdb)

The fix is to loop through the pci addresses in the avf interface pool
and to reject the duplicate.

Type: improvement

Signed-off-by: Steven Luong <sluong@cisco.com>
Change-Id: I4ed6fb630fb11982d85c5bb325d9f0d6beeaf023