volume: use qemuimg and remove custom createVolume

This patch substitutes a custom createVolume function with the more
standard implementation in qemuimg module.
As a side-effect the 'preallocate' flag for the clone method became
useless and it has been removed.

Change-Id: I960276ec6bd654be4f427425c95603189c0d6a99
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26871
Reviewed-by: Dan Kenigsberg <danken@redhat.com>