raw-posix: handle > 512 byte alignment correctly

Replace the hardcoded handling of 512 byte alignment with bs->buffer_alignment
to handle larger sector size devices correctly.

Note that we can not rely on it to be initialize in bdrv_open, so deal
with the worst case there.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
