Failed to install to /dev/zd0

[Yuuki Galaxy]

Process to produce the bug

```
# boot with ubuntu-20.04-live-server-amd64.iso
sudo apt install zfsutils-linux
# create bpool and rpool follow <https://git.launchpad.net/ubiquity/tree/scripts/zsys-setup>
zfs create -V 16G rpool/USERDATA/zInsVol
# use gdisk to make one GPT partition as /dev/zd0p1.

```

The installer will say `Probing for devices to install to failed`.

Crash log

```
 2020-06-03 05:54:52,845 ERROR root:39 finish: subiquity/Filesystem/_probe/probe_once: FAIL: Invalid dep_id (disk-zd0) not in
 storage config
 2020-06-03 05:54:52,845 ERROR block-discover:151 block probing failed restricted=False
 Traceback (most recent call last):
   File "/snap/subiquity/1772/lib/python3.6/site-packages/subiquity/controllers/filesystem.py", line 144, in _probe
     self._probe_once_task.task, 15.0)
   File "/snap/subiquity/1772/usr/lib/python3.6/asyncio/tasks.py", line 358, in wait_for
     return fut.result()
   File "/snap/subiquity/1772/lib/python3.6/site-packages/subiquity/controllers/filesystem.py", line 124, in _probe_once
     self.model.load_probe_data(storage)
   File "/snap/subiquity/1772/lib/python3.6/site-packages/subiquity/models/filesystem.py", line 1564, in load_probe_data
     self.reset()
   File "/snap/subiquity/1772/lib/python3.6/site-packages/subiquity/models/filesystem.py", line 1285, in reset
     self._probe_data)["storage"]["config"]
   File "/snap/subiquity/1772/lib/python3.6/site-packages/curtin/storage_config.py", line 1320, in extract_storage_config
     tree = get_config_tree(cfg.get('id'), final_config)
   File "/snap/subiquity/1772/lib/python3.6/site-packages/curtin/storage_config.py", line 275, in get_config_tree
     for dep in find_item_dependencies(item, sconfig):
   File "/snap/subiquity/1772/lib/python3.6/site-packages/curtin/storage_config.py", line 245, in find_item_dependencies
     _validate_dep_type(item_id, dep_key, dep, config)
   File "/snap/subiquity/1772/lib/python3.6/site-packages/curtin/storage_config.py", line 193, in _validate_dep_type
     'Invalid dep_id (%s) not in storage config' % dep_id)
 ValueError: Invalid dep_id (disk-zd0) not in storage config
 2020-06-03 05:54:52,846 DEBUG subiquity.core:421 generating crash report
 2020-06-03 05:54:52,847 INFO subiquity.core:438 saving crash report 'block probing crashed with ValueError' to /var/crash/1591163692.846989393.block_probe_fail.crash

```