Rough-in a dummy handler

dummy.c is a test handler. So far all it can do is load properly.

Signed-off-by: Andy Grover <agrover@redhat.com>
Add cfgstring to struct tcmu_device

Thus we no longer need to pass it to handlers (already pass that struct).

Implement find_handler. Call handler->open() from add_device, and ->close()
from remove_device.

Pass cfgstring to add_device().

Free dirent_list in open_devices all at the end.

Signed-off-by: Andy Grover <agrover@redhat.com>