VMware: Support OVA container format

Currently, the VMware is not supporting images with "ova" container
format. This spec adds support for it by extracting the root disk from
the OVA and use it to boot the VM.

Previously-approved: juno

Change-Id: I8704c697cafbe7f17cde6a3eadcad5e9d21e35b3
Merge "VMware: Support OVA container format"