storage: Search only the current image for children

The getChildren method of FileVolume is currently searching all images
in the storage domain for children.  A glob of all metadata files in the
SD is cached and then passed to grep to look for the child volumes.  The
problem is that deleteVolumes takes an exclusive lock on the image
namespace only.  If deletes are active on other images at the same time
we'll get errors with missing files.

The code attempts to report the children of a template base volume.
However, this is not needed now and is not expected to be required in
the future.  The getVolsOfImage API should be used for templates.
Therefore, we can fix this race by limiting the search for children to a
single image.

Change-Id: I2ef9012cee3a8cb891926510c10ecc47b7cddaa1
Signed-off-by: Adam Litke <alitke@redhat.com>
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1069610
Reviewed-on: http://gerrit.ovirt.org/35096
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>