vnet: allow format deleted swifidx

This patch prevents the sw interfaces format
function to fail when the interface was deleted.
It also prints the swifindex alongside the 'DELETED'
keyword.

Printing deleted swifindex should not happen, but it is still
helpful to have these safeguards for troubleshooting in the case
invariants get corrupted (e.g. fib entry refcounts, ...)

Type: improvement

Change-Id: I66711049db2eebe0ad17e37c3a260ac81d1e5134
Signed-off-by: Nathan Skrzypczak <nathan.skrzypczak@gmail.com>