Controller.py: add driver_iothreads for scsi controller

[meinaLi]

Add new iothread and queues to driver element of scsi controller

[meinaLi]

Test result for fetch_attrs:

>>> from virttest.libvirt_xml.devices import controller
>>> controller_dev = controller.Controller()
>>> controller_dev.xml = """
... <controller type='scsi' index='1' model='virtio-scsi'>
...       <driver queues='2'>
...         <iothreads>
...           <iothread id='4'>
...             <queue id='0'/>
...           </iothread>
...           <iothread id='2'>
...             <queue id='1'/>
...           </iothread>
...         </iothreads>
...       </driver>
...       <address type='pci' domain='0x0000' bus='0x00' slot='0x0c' function='0x0'/>
...     </controller>
... """
>>> controller_dev.fetch_attrs()
{'driver_iothreads': {'iothread': [{'queue': [{'id': '0'}], 'id': '4'}, {'queue': [{'id': '1'}], 'id': '2'}]}, 'model': 'virtio-scsi', 'address': {'attrs': {'type': 'pci', 'domain': '0x0000', 'bus': '0x00', 'slot': '0x0c', 'function': '0x0'}, 'type_name': 'pci'}, 'type_name': 'scsi', 'driver': {'queues': '2'}, 'type': 'scsi', 'index': '1'}

[meinaLi]

Test result for setup_attrs():

>>> from virttest.libvirt_xml.devices import controller
>>> controller_dev = controller.Controller()
>>> controller_dict = {'driver_iothreads': {'iothread': [{'queue': [{'id': '0'}], 'id': '4'}, {'queue': [{'id': '1'}], 'id': '2'}]}, 'model': 'virtio-scsi', 'address': {'attrs': {'type': 'pci', 'domain': '0x0000', 'bus': '0x00', 'slot': '0x0c', 'function': '0x0'}, 'type_name': 'pci'}, 'type_name': 'scsi', 'driver': {'queues': '2'}, 'type': 'scsi', 'index': '1'}
>>> controller_dev.setup_attrs(**controller_dict)
>>> controller_dev
{'xmltreefile': None, 'validates': None, 'xml': <_io.FileIO name='/tmp/xml_utils_temp_rcug4_0d.xml' mode='rb+' closefd=True>, 'virsh': <module 'virttest.virsh' from '/var/ci/libvirt-ci/runtest/avocado-vt/avocado-vt/virttest/virsh.py'>, 'device_tag': 'controller'}

# cat /tmp/xml_utils_temp_rcug4_0d.xml
<controller type="scsi" model="virtio-scsi" index="1"><driver queues="2"><iothreads><iothread id="4"><queue id="0" /></iothread><iothread id="2"><queue id="1" /></iothread></iothreads></driver><address type="pci" domain="0x0000" bus="0x00" slot="0x0c" function="0x0" /></controller>