devices: vhost-user crashes displaying show trace for deleted interface

devices: vhost-user crashes displaying show trace for deleted interface

After the trace is collected and if the interface is then deleted, show
trace may crash for the debug image. This is due to the additional check
in pool_elt_at_index() to make sure that the block is not free.

The fix is to do the check in vhost format trace and return "interface deleted"

Type: fix

Signed-off-by: Steven Luong <sluong@cisco.com>
Change-Id: I0744f913ba6146609663443f408d784067880f93
(cherry picked from commit 5cd987dda679fe50b9cd7a834bb9162db39ade78)