[Stgt-devel] add it_nexus support

[FUJITA Tomonori]

tgt uses scsi host_no as it_nexus. This works for ibmvstgt because it
creates one scsi host for one it_nexus. However, this doesn't work for
some target drivers.

I added it_nexus support and modified ibmvstgt to use it. It needs to
be polished before it goes to scsi-ml. But I applied this to my trees
since this has been long overdue and some people want to try it, I
guess.

The user/kernel interface was changed so get and compile the latest
user-space and kernel-space code.