blockVolume: remove inexplicable fixme comment

There's no apparent reason for this fixme. Since the line is raising an
exception the "fix me" string appears in the tracebacks making our code
appear sloppy.

I searched for other inexplicable fixmes but it seems that this was the
only one.

Change-Id: I39c5a2034fdc7e5300259a2c5a25b0aed7f679b6
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/33949
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>