fix: Avoid implicit storage probing

[joseivanlopez]

Problem

Storage devices were implictly probed. As side effect, the UI logs some errors when it asks for the available devices for first time.

Trace:

• The Storage service starts and exports the D-Bus object /org/opensuse/Agama/Storage1.
• The method #available_devices is called in order to populate the corresponding D-Bus property.
• The system is automatically probed as side effect of calling to Y2Storage::DiskAnalizer.
• The SIDs of the reported available devices will not exist anymore once the storage probing is explicitly requested by the Manager service.

Solution

Avoid implicit probing.

[coveralls]

coverage: 69.869% (+0.006%) from 69.863%
when pulling 0b016ed on joseivanlopez:fix-storage-service
into 0a10e9a on openSUSE:master.