ESXi rejects streamOptimized VMDKs built with QEMU < 2.5.1

[glennmatthews]

A fun corner case in "streamOptimized" VMDK support:

• The VMDK file header includes a version number.
• Recent versions of ESXi will outright reject streamOptimized VMDKs that
  do not self-report as using the "version 3" VMDK format. (Not a supported disk format (sparse VMDK version too old))
• Although QEMU 2.1.0 and later can output to streamOptimized VMDK,
  only in 2.5.1 and later does it produce "version 3" streamOptimized VMDKs.
• Since version 1.1.2, COT has preferred QEMU 2.1.0 over vmdktool, but if QEMU > 2.1.0 but less than 2.5.1, it is producing version 1 VMDKs which are not usable with recent ESXi.

COT should therefore prefer QEMU over vmdktool only if QEMU version is at least 2.5.1, not 2.1.0.