storage: fix omitted slash in gluster volume URI

storage: fix omitted slash in gluster volume URI

When doing 'virsh vol-dumpxml' on a gluster pool's volume, the
resulting URI incorrectly omitted a slash between hostname and
path:  gluster://192.168.122.206rhsvol1/fedora-19.img

This is fallout from me rebasing earlier versions of my patch
that ended up as commit efee1af; I had originally played with
always requiring the gluster volume to have a leading slash,
but it was easier to use the gluster API if the gluster volume
name was guaranteed to have no slash.  While I got the URI of
the pool correct, I forgot to fix the URI of a libvirt volume.

* src/storage/storage_backend_gluster.c
(virStorageBackendGlusterRefreshVol): Use correct starting point
since uri construction requires leading slash.

Signed-off-by: Eric Blake <eblake@redhat.com>