[SCSI] ibmvstgt: set up scsi_host properly before __scsi_alloc_queue

Before calling __scsi_alloc_queue, scsi_host->shost_gendev.parent must
be initialized properly.

This patch moves __scsi_alloc_queue after scsi_add_host (like
initiator drivers do).

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>