v2v: Add documentation for diagnosing RHEV-M import problems.
v2v: Generate qemu-0.10 compatible qcow2 files for RHEV-M.

RHEV nodes running on RHEL 6 cannot read the new qcow2 compat=1.1
(a.k.a qcow2 v3) format.

Thanks: Junqin Zhou, Tingting Zheng