test/vhost: remove hardcoded vhost name id

This line called vhost_kill with 2 arguments:
$1 - string "0"
$2 - string "vhost_name"

Problem was that vhost_kill only accepts 1 argument, which
is "vhost_name". This was not noticed because in seems in
most of our tests we just use the name "0".
This would fail to kill a vhost instance in case it would
be named anything else than "0"

Change-Id: I635c5b92673abb7350d8bd6c636f6b794651f7ac
Signed-off-by: Karol Latecki <karol.latecki@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/468201


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>