homed: make clean that --storage=directory --image-path=/dev/some-block-device is...

homed: make clean that --storage=directory --image-path=/dev/some-block-device is not supported

The directory backend needs a file system path, and not a raw block
device. That's only supported for the LUKS2 backend.

Let's make this clearer in the man page and also generate a better error
message if attempted anyway.

Fixes: #17068
(cherry picked from commit f9d525ae558105bf7fd77ad76e4fdb135bb9f634)