baremetal: Make volume driver use a correct source device

attach_volume() have to take the source device path from returning of
LibVirtVolumeDriver.connect_volume() rather than connection_info.

It also adds a test for attach_volume().

Change-Id: I3bee80560342104e2df614066301171624497a04
Closes-Bug: #1208707
Merge "baremetal: Make volume driver use a correct source device"