libvirt_vm: Introduce a method named remove_with_storage.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
How about moving this part to `remove()` function, and add a parameter `remove_storage=False` for judging if the storage should be removed.Then use `libvirt_vm.VM(clone_name, params, None, None).remove(remove_storage=True)` in https://github.com/autotest/tp-libvirt/issues/250