qemu-nbd: pass structure into nbd_client_thread instead of plain char*

qemu-nbd: pass structure into nbd_client_thread instead of plain char*

We are going to pass additional flag inside next patch.

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Eric Blake <eblake@redhat.com>
CC: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
CC: <qemu-stable@nongnu.org>
Message-ID: <20230717145544.194786-2-den@openvz.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
(cherry picked from commit 03b67621445d601c9cdc7dfe25812e9f19b81488)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>