run.py: Add aio=native for virtio-blk device

aio=native makes QEMU to use Linux native implementation of aio instead
of emulated thread pool which improves tst-bdev-write.so performance:

  blk, aio=threads
  Wrote 691.910 MB in 10.03 s
  Wrote 590.020 MB in 10.00 s
  Wrote 605.578 MB in 10.01 s
  Wrote 662.828 MB in 10.01 s
  Wrote 624.762 MB in 10.01 s

  blk, aio=native
  Wrote 789.566 MB in 10.00 s
  Wrote 744.691 MB in 10.00 s
  Wrote 537.125 MB in 10.00 s
  Wrote 732.230 MB in 10.00 s
  Wrote 683.383 MB in 10.00 s

  scsi, aio=threads
  Wrote 200.863 MB in 10.02 s
  Wrote 193.758 MB in 10.02 s
  Wrote 193.680 MB in 10.03 s
  Wrote 195.211 MB in 10.03 s
  Wrote 190.762 MB in 10.02 s

  scsi, aio=native
  Wrote 414.344 MB in 10.05 s
  Wrote 483.148 MB in 10.00 s
  Wrote 537.477 MB in 10.01 s
  Wrote 477.727 MB in 10.01 s
  Wrote 462.805 MB in 10.01 s

Signed-off-by: Asias He <asias@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
Update pre-requisites in README.md

We were missing the prerequisite qemu-img (needed for qemu-nbd, used
during our build) and of course qemu-system-x86 (we run the guest as
part of the build, to have it write files into the ZFS image).

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>