systemd-nspawn fails when a devtmpfs entry is present in /etc/fstab:

   mknod(/var/lib/leapp/scratch/mounts/root_/system_overlay/dev/null) failed: File exists

Ignoring such vfstype fixes the issue rhbz#2215027

(cherry picked from commit 6e470f4)

Author: Christophe Besson

repos/system_upgrade/common/libraries/overlaygen.py

@@ -10,7 +10,7 @@
 from leapp.libraries.stdlib import api, CalledProcessError, run
 
 
-OVERLAY_DO_NOT_MOUNT = ('tmpfs', 'devpts', 'sysfs', 'proc', 'cramfs', 'sysv', 'vfat')
+OVERLAY_DO_NOT_MOUNT = ('tmpfs', 'devtmpfs', 'devpts', 'sysfs', 'proc', 'cramfs', 'sysv', 'vfat')
 
 # NOTE(pstodulk): what about using more closer values and than just multiply
 # the final result by magical constant?... this number is most likely going to