Bug 2449340

Anaconda crashes when finalizing installation of custom installation with /home on separate drive, seems to fail to get separate drive.

Message error is "Le programme d'installation ne peut pas continuer en raison d'une erreur critique : Installation of the system failed: Enregistrement des fichiers de configuration et kickstart", details given are "org.fedoraproject.Anaconda.Error: 'NoneType' object has no attribute 'path'".

Looking at `/tmp/journal.log`, it seems Anaconda.Modules.Storage fails to find the disk that it just managed to format properly earlier:

```
Mar 19 22:37:52 localhost-live org.fedoraproject.Anaconda.Modules.Storage[5450]: DEBUG:blivet:               DeviceTree.get_device_by_device_id: device_id: BTRFS-00ed7298-89f5-4ee4-9044-c0da8c17b9dd ; incomplete: False ; hidden: False ;
Mar 19 22:37:52 localhost-live org.fedoraproject.Anaconda.Modules.Storage[5450]: DEBUG:blivet:               DeviceTree.get_device_by_device_id returned None

```

Custom install was basically:
- system SSD
  - 2 GiB /boot/efi
  - 16 GiB swap
  - everything else as btrfs /
- home SSD, btrfs

Partition scheme had already been created previously, but partition were formatted as part of the install process.

Full log will be included as attachment.

Reproducible: Always

Steps to Reproduce:
1. boot to live-USB
2. begin the graphical install process
3. when prompted for the install target, select both disks, then choose to manually assign mount points (see screenshot 1)
4. assign mount points with /home on the second disk (see screenshot 2)
Actual Results:
Installer crash before creating the user, subsequent boot bring up the user creation process for a first boot with no user. Also settings (specific keyboard layout) are not properly set (as that require the user existing at the end of the installation process). 

Expected Results:
User should have been properly created.