migration: fix SEEK_CUR offset calculation in qio_channel_block_seek

migration: fix SEEK_CUR offset calculation in qio_channel_block_seek

The SEEK_CUR case in qio_channel_block_seek was incorrectly using the
'whence' parameter instead of the 'offset' parameter when calculating the
new position.

Fixes: 65cf200a51 ("migration: introduce a QIOChannel impl for BlockDriverState VMState")
Signed-off-by: Marco Cavenati <Marco.Cavenati@eurecom.fr>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Message-ID: <20250326162230.3323199-1-Marco.Cavenati@eurecom.fr>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
(cherry picked from commit c0b32426ce56182c1ce2a12904f3a702c2ecc460)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>