Avoid running qemu-img when hypervisor is specified as esxi.

This is shown in the logs before this patch:

```
qemu-img: Could not open '[0] vm1/vm1.vmdk': No such file or directory
```

And the result from running virt.get_disks vm1 was before this patch:

```
build04:
    ----------
    sda:
        ----------
        file:
            [0] vm1/vm1.vmdk
        image:
            Does not exist
```

After the patch is applied:

```
build04:
    ----------
    sda:
        ----------
        file:
            [0] vm1/vm1.vmdk
```
Remove print()
Unify name: emulator vs hypervisor
Clarify and make connection settings more flexible for libvirt

Previously you could specify a hostname or IP in the minion configuration. It was not flexible enough. Now you can either specify an explicit libvirt URI, or a libvirt connection alias.

Note: this is an incompatible change to #6672 when it comes to the configuration. The connection string can no longer contain a hostname.

 - http://libvirt.org/drvesx.html#uriformat
 - http://libvirt.org/uri.html#URI_config
Merge branch 'develop' into esxi
KVM and qemu both use qemu-img
Merge pull request #6740 from holmboe/esxi

Changes to virt.py for ESXi support