block: move bdrv_qiov_is_aligned to file-posix

There is only user of bdrv_qiov_is_aligned(), so move the alignment
function to there and make it static.

Signed-off-by: Keith Busch <kbusch@kernel.org>
Message-Id: <20220929200523.3218710-2-kbusch@meta.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>