vhost-user-blk: split vhost_user_blk_sync_config()

vhost-user-blk: split vhost_user_blk_sync_config()

Split vhost_user_blk_sync_config() out from
vhost_user_blk_handle_config_change(), to be reused in the following
commit.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Acked-by: Raphael Norwitz <raphael@enfabrica.net>
Message-Id: <20240920094936.450987-3-vsementsov@yandex-team.ru>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>