volume: remove unused imgPath argument from clone

This patch removes the unused imgPath argument from clone and all its
related methods.

Change-Id: I40c9b1208fcba95c7af2deb10d45ed8a1042e9b5
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27747
Reviewed-by: Dan Kenigsberg <danken@redhat.com>