fileVolume: create file volume with correct mode

This patch removes a race in the volume file permissions between
creation and file_setrw. Now there's no window of time where the
volumes may have permissions that are not restrictive enough.
The correct mode is now also explicit using the VOLUME_PERMISSIONS
constant.

Change-Id: I38aabad8d3662d6de0a24c33a0f73976a3e98ce3
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27593
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>