disk: support driver iothreads elements

[dzhengfy]

This is to support the new elements in disk as below:

Signed-off-by: Dan Zheng [email protected]

[dzhengfy]

from virttest.libvirt_xml import vm_xml vmxml = vm_xml.VMXML.new_from_dumpxml('vm1') vmxml.fetch_attrs() { 'devices': [{'path': '/usr/libexec/qemu-kvm'}, {'address': {'attrs': {'type': 'pci', 'domain': '0x0000', 'bus': '0x04', 'slot': '0x00', 'function': '0x0'}, 'type_name': 'pci'}, 'driver_iothreads': {'iothread': [{'queue': [{'id': '1'}], 'id': '2'}, {'queue': [{'id': '0'}, {'id': '2'}], 'id': '3'}]}, 'backingstore': {}, 'type_name': 'file', 'device': 'disk', 'target': {'dev': 'vda', 'bus': 'virtio'}, 'driver': {'name': 'qemu', 'type': 'qcow2', 'queues': '3'}, 'alias': {'name': 'virtio-disk0'},