fw4: simplify `is_loopback_dev()`

fw4: simplify `is_loopback_dev()`

Use `fs.readfile()` to simplify the code reading flag values from sysfs.

Also add a mock implementation of `fs.readfile()` using the same mock
data files as `fs.open()`.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>