Port coverage API into v3 part2

This patch contains the changes required to adapt the hypervisor
extension and the corresponding unittest to the v3 framework.

And adding '_import_coverage' for ensure loading coverage module from
python-coverage.

Partially implements bp v3-api-extension-versioning

Change-Id: I6a2dbf00d09132b767cd7972f68cb752b5961796
Merge "Port coverage API into v3 part2"