Move copy_volume function to volume/utils.py.

There are several copy-pastes of running dd, so moved the most
correct one (LVM) to volume/utils.py, and also updated
BlockDeviceDriver to use it. Other drivers (e.g., NFS, GPFS, Scality)
should be updated as well. Volume migration for detached volumes
is a future use case.

Change-Id: I56c64c94eccf0a311e6f3d611738ad0403351971
Merge "Move copy_volume function to volume/utils.py."