v2v: esx: Add note to documentation about being asked for the password twice.
Add support for qemu copy_on_read feature.

When drives are added with this flag, reads from the backing file are
copied into the overlay, improving performance of reads to the same
area of disk (at the expense of local storage).  This is excellent for
reading remote / slow sources like HTTP.
v2v: Use copy_on_read in the overlays.

To speed up conversions by storing everything we read during
conversion in the overlay instead of having to reread it from the
remote server during copying.

Thanks: Matthew Booth.
v2v: Output to OpenStack Glance (-o glance option).