bug fix: guest iface does not return model if no model is found

On some distros, libvirt will not set model automatically.
if no model is found,  guest iface will not return this field.

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>
Signed-off-by: Aline Manera <alinefm@br.ibm.com>
support create/delete VMIface: update model

attach a network interface to vm in model
detach a network interface from vm in model

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>
support create/delete VMIface: update mockmodel

attach a network interface to vm in mockmodel
detach a network interface from vm in mockmodel

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>
support create/delete VMIface: update API.json

add vmifaces_create parameters verification in API.json

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>
support create/delete VMIface: update testcase

update tests/test_model.py and tests/test_rest.py

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>
PEP8 cleanup and bug fix for src/kimchi/utils.py

This patch makes src/kimchi/utils.py pep8 clean, and fix the
wrong var reference of 'storage_server'.

Signed-off-by: Mark Wu <wudxw@linux.vnet.ibm.com>