Add a -n option to skip volume creation on qemu-img convert.
This is useful for targets such as rbd / ceph, where the
target volume may already exist; we cannot always rely on
qemu-img convert to create the image, as dependent on the
output format, there may be parameters which are not possible
to specify through the qemu-img convert command line.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Alexandre Derumier <aderumier@odiso.com>
Signed-off-by: Alex Bligh <alex@alex.org.uk>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>