Pass 'verbose' to nbd_client_thread() inside NbdClientOpts which looks
a little bit cleaner and make it bool as it is used as bool actually.

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Eric Blake <eblake@redhat.com>
CC: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Message-ID: <20230717202520.236999-1-den@openvz.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>