ibmvstgt: remove

The IBM virtual SCSI protocol has been obsoleted by ibmvfc, and there
are no reported of the driver left.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>