ceph-volume: fix loop devices support

ceph-volume: fix loop devices support

This commit updates the `is_device` function to correctly handle
loop devices.
The function now validates loop devices when they are explicitly
allowed, by checking their type (`loop`) in addition to `disk`
and `mpath`.

Changes include:
  - Extending the type check to include `loop` in the list of
    supported device types.
  - Enhancing the docstring for better documentation of the
    function's purpose and behavior.

These changes ensure that loop devices are properly recognized
and handled when configuring OSDs in ceph-volume.

Fixes: https://tracker.ceph.com/issues/69432
Signed-off-by: Guillaume Abrioux <gabrioux@ibm.com>