misc: binary-api sw_interface_vhost_user_dump is broken

Recent modificaton to vhost_user APIs for typing which added a sw_if_index
filter to the API sw_interface_vhost_user_dump with the default value -1 to
mean all interfaces. But the default is not set from api_format.c, causing the
binary-api command for sw_interface_vhost_user_dump to display nothing.

Also missing is the proper display on custom dump for the aformentioned API.

Type: fix

Signed-off-by: Steven Luong <sluong@cisco.com>
Change-Id: I05546e2318165af7531977fbed66ea7224c4a7ce