raw-posix: Fail gracefully if no working alignment is found If qemu couldn't find out what O_DIRECT alignment to use with a given file, it would run into assert(bdrv_opt_mem_align(bs) != 0); in block.c and confuse users. This adds a more descriptive error message for such cases. Signed-off-by: Kevin Wolf <kwolf@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>