Libvirt: volume driver set correct device type

Due to a typo, libvirt volume driver was assigning the device type
passed to the connect_volume method to the wrong attribute of the
LibvirtConfigGuestDisk class (non existing device_type which is
completely disregarded, instead of the expected source_device), which
caused the rendered XML to always have the device set to 'disk'. This
patch fixes that issue.

Also adds the 'lun' type to the list of supported device_types since it
is supported by libvirt but was missed in the previous refactoring.

Tests are added to prevent future regressions.

Closes-bug: #1223890

Change-Id: Iee160afcc23f762a752df1ca20372944d5100291
Merge "Libvirt: volume driver set correct device type"