The z Initial Program Loader (zipl) when creating the bootmap in
bootmap_creat (src/zipl/bootmap.c) wants to create a device node
via misc_temp_dev (bootmap_create:1141) for the device that it
is installing the bootloader to[1]. Currently access to loopback
devices is allowed from within the container (it is used to mount
the image), but only read/write access. On s390x also allow the
creation of device nodes, so zipl can do its work and install
the bootloader stages on the "disk".
[1] zipl source at commit dcce14923c3e9615df53773d1d8a3a22cbb23b96