Virt: remove unnecesary return code

The methods attach_volume, detach_volume and swap_volume should
not return anything.

The purpose of the patch set is to have a consistent behaviour across
drivers for the aforementioned methods.

Change-Id: I381ede44d71cd5f6ce98804853e4e8f587b46c99
Merge "Virt: remove unnecesary return code"