libvirt.tests: Add a case for iscsi network/volume type disk hotplug testing

This case including:
```
virtual_disks.at_dt_iscsi_disk.positive_test.hot_plug.network_disk.ipv4_virtio_bus
virtual_disks.at_dt_iscsi_disk.positive_test.hot_plug.network_disk.ipv6_virtio_bus
virtual_disks.at_dt_iscsi_disk.positive_test.hot_plug.network_disk.usb_bus
virtual_disks.at_dt_iscsi_disk.positive_test.hot_plug.network_disk.readonly_mode
virtual_disks.at_dt_iscsi_disk.positive_test.hot_plug.network_disk.private_secret
virtual_disks.at_dt_iscsi_disk.positive_test.hot_plug.network_disk.public_secret
virtual_disks.at_dt_iscsi_disk.positive_test.hot_plug.volume_disk.ipv4_host_source_mode
virtual_disks.at_dt_iscsi_disk.positive_test.hot_plug.volume_disk.ipv6_host_source_mode
virtual_disks.at_dt_iscsi_disk.positive_test.hot_plug.volume_disk.direct_source_mode
virtual_disks.at_dt_iscsi_disk.positive_test.hot_plug.volume_disk.save_domain
virtual_disks.at_dt_iscsi_disk.positive_test.hot_plug.volume_disk.snapshot_domain
virtual_disks.at_dt_iscsi_disk.positive_test.cold_plug.network_disk.ipv4_virtio_bus
virtual_disks.at_dt_iscsi_disk.positive_test.cold_plug.network_disk.ipv6_virtio_bus
virtual_disks.at_dt_iscsi_disk.positive_test.cold_plug.network_disk.usb_bus
virtual_disks.at_dt_iscsi_disk.positive_test.cold_plug.network_disk.readonly_mode
virtual_disks.at_dt_iscsi_disk.positive_test.cold_plug.network_disk.private_secret
virtual_disks.at_dt_iscsi_disk.positive_test.cold_plug.network_disk.public_secret
virtual_disks.at_dt_iscsi_disk.positive_test.cold_plug.volume_disk.ipv4_host_source_mode
virtual_disks.at_dt_iscsi_disk.positive_test.cold_plug.volume_disk.ipv6_host_source_mode
virtual_disks.at_dt_iscsi_disk.positive_test.cold_plug.volume_disk.direct_source_mode
virtual_disks.at_dt_iscsi_disk.positive_test.cold_plug.volume_disk.save_domain
virtual_disks.at_dt_iscsi_disk.positive_test.cold_plug.volume_disk.snapshot_domain 
```

Signed-off-by: Yanbing Du <ydu@redhat.com>