powervm: remove powervm virt driver from nova

IBM's strategic direction for OpenStack powervm support
is to support powervc, and therefore support is being
pulled out for the existing powervm IVM/VIOS driver.

DocImpact

Change-Id: I9b7887c1914def145e87cf20dbd0ec65cb0fe987
Merge "powervm: remove powervm virt driver from nova"