block: make qiov_is_aligned() public

The qiov_is_aligned() function checks whether a QEMUIOVector meets a
BlockDriverState's alignment requirements.  This is needed by
virtio-blk-data-plane so:

1. Move the function from block/raw-posix.c to block/block.c.
2. Make it public in block/block.h.
3. Rename to bdrv_qiov_is_aligned().
4. Change return type from int to bool.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
