Is this Solaris 11 or Solaris 11.1?
I tried to reproduce this and haven't seen a misbehaving iSCSI target preventing a reboot.
Is the device that is providing the target faulted or in any way unavailable?
I used to see similar messages on Solaris 10 U10 when an iSCSI LUN was backing a ZFS pool.
I raised a SR but I opted out for an IDR since then.
I don't know if it was fixed for Solaris 11.
There are still a few issues with Solaris 11.1 and iSCSI LUN and rebooting. I know they are working on fixes.
A possible workaround is to use the reboot command and NOT the init command. At least when you use the reboot the command the system will reboot and not hang.
If this is not your problem then it may be a different problem.