host net-io: update API.md

update API.md, get host net_throughput

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>
host net-io: update model and mockmodel

use psutil to get net io counter
get random io rate in mockmodel

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>
host net-io: update test case

update test_model and test_rest

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>