New tests to stoqlib/domain/devices.py

This patch implements new cases of test to domain/devices.py

Change-Id: I04d773de037e1dc7abfdc561d973dcdab6185ee9
Update tests to stoqlib/domain/image.py

This patch apply modifications in cases of test of domain/test/test_image.py

Change-Id: I6d0df5e7cf4cab8e0ef939ce73b881993655b215