add qemu-img convert -C option (skip target volume creation)

Code:

Author: Alexandre Derumier <aderumier@odiso.com>
Signed-off-by: Alexandre Derumier <aderumier@odiso.com>
Signed-off-by: Alex Bligh <alex@alex.org.uk>

Documentaton:

Author: Alex Bligh <alex@alex.org.uk>
Signed-off-by: Alex Bligh <alex@alex.org.uk>