target: add a new add_wwn_groups fabrics method

We need to have the WWN fully initialized before addig default groups to it,
so add a new method to add these groups after the WWN has been initialized.
Also remove the default groups in the core while we're at it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>