If you google it you'll find that hyper-v makes up around 18 - 28% based on who doing the counting
Both your articles are from 2009 or earlier. The gartner article put VMWare's market share at 85%. That leaves 15% for the rest. Best case, in 2009, MS had < 15%.
Is there updated data from Gartner?
The dabcc article (who are they first of all) explain their counting at the end. They count shipments for virtualization software, not usage. So, every copy of Windows Server 2008 counted as a Hyper-V purchase. Also, that article is from 2008. It gave VMWare a 78% market share. The IDC article, a year later gave VMWare an 85% market share.
From the provided articles I see a declining trend in Hyper-V usage.