Append ESX driver to perform OVA install operation

This patch provides implementation support to define new domain
using given OVA package for an ESXi server.




As per vSphere API documentation, the OVA installation consists of
following steps:
1. Run OVF descriptor against hypervisor to check if it can be
   deployed.
2. Obtain ImportSpec object from hypervisor defining new domain's
   specifications such as: virtual hrdware detials, networks etc.
3. Create virtual machine entity on the hypervisor.
4. Upload virtual disk contained inside OVA package using lease
   obtained from step 3.