tcmu: add helper to check if dev was configured

tcmu: add helper to check if dev was configured

This adds a helper to check if the dev was configured. It
will be used in the next patch to prevent updates to some
config settings after the device has been setup.

Signed-off-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
Orabug: 25395066
(cherry picked from commit 972c7f167974fa41ea8a2eed4b857cc59f59c42c)
Signed-off-by: Kyle Fortin <kyle.fortin@oracle.com>
Reviewed-by: Shan Hai <shan.hai@oracle.com>
Conflicts:
	drivers/target/target_core_user.c