lib: add create function to the qemuImg module

Change-Id: I69522cd6231e511d891815248fab9a15cbc987e5
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/18935
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Yeela Kaplan <ykaplan@redhat.com>