Able to connect to ESXi.

Preliminary support for libvirtd to connect to ESXi.

Tested to work with:

 virt.list_vms
 virt.vm_info <vm>

It will fall back to the default qemu connection, unless configured with:

```yaml
virt:
  hypervisor: esxi
  hostname: 192.168.9.9
```
Merge pull request #6672 from holmboe/esxi

Connect to ESXi