domain: unify the createVolume method

Change-Id: I03eeaa2561fd13b4094634f5cfeee644d6396a18
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/13329
Reviewed-by: Ayal Baron <abaron@redhat.com>
Reviewed-by: Deepak C Shetty <deepakcs@linux.vnet.ibm.com>
Reviewed-by: Yeela Kaplan <ykaplan@redhat.com>
Reviewed-by: Saggi Mizrahi <smizrahi@redhat.com>