aio_writev: fall back to slow path if iovec is not sectorsize-aligned
However, for most disk-like devices the I/O will eventually fail anyway.
physio has the same constraints as aio_qbio. I think it might work for
zols, though; I need to add a test for that.
This commit also adds a few more test cases.
This commit add some dtrace probes, too, though I might remove those later.