vmiface update support: update API.md

update API.md and API.json

Test this patch by:

$ curl -u <user>:<password> -H "Content-Type: application/json" \
> -H "Accept: application/json"
> http://localhost:8000/vms/<vmname>/ifaces/<mac> \
> -X PUT -d '
> {
>   "model":"virtio",
>   "network":"default"
> }'

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

For network source, change will be on the active VM instance and
persisted VM configuration.

For model, change will be on the active VM instance only

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

update mockmodel

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>
vmiface update support: update test case

update test_model.py and test_rest.py

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