target: Better handling of AllRegistrants reservations

Fix AllRegistrants reservations in register_and_move() function where
the code didn't handle all of the registered devices as reservation
holders, resulting in the wrong warning message being displayed.

At the same time, introduce a helper function named
'is_reservation_holder()' that properly checks if a device is a
reservation holder, taking into account the reservation type. This
function cleans up the code and improves readability.

Signed-off-by: Ilias Tsitsimpis <iliastsi@arrikto.com>
Signed-off-by: Vangelis Koukis <vkoukis@arrikto.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>