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

This case including:
    IPv4/IPv6 protocol iscsi disk hotplug
    usb target bus iscsi disk hotplug
    readpnly iscsi disk hotplug
    private/public secret iscsi disk hotplug

Signed-off-by: Yanbing Du <ydu@redhat.com>