Unify produceVolume

Unify produceVolume function and make it use getVolumeClass
so that subclasses can override getVolumeClass to specify
their own Volume and ensure that produceVolume works
properly with subclass supplying their own volumeclass.

Change-Id: I367582b2741dc0276ec64d8d13fa1e4953f0314b
Signed-off-by: Deepak C Shetty <deepakcs@linux.vnet.ibm.com>
Reviewed-on: http://gerrit.ovirt.org/8115
Reviewed-by: Dan Kenigsberg <danken@redhat.com>