sd: IncorrectFormat cannot be decoded

If the volFormat and/or preallocated parameters are not within the
supported values it's most likely impossible to try and convert them
into names.

Change-Id: Ic03271be3f58b8a68421b4736c9fe403b2448265
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28647
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Allon Mureinik <amureini@redhat.com>