tcmu: Make dev_config configurable

This allows for userspace to change the device path after
it has been created. Thus giving the user the ability to change
the path. The use case for this is to allow for virtual optical
to have media change.

Signed-off-by: Bryant G. Ly <bryantly@linux.vnet.ibm.com>
Reviewed-By: Mike Christie <mchristi@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>