Changes to virt.py for ESXi support

Contains the following commits:

 - Clarify and make connection settings more flexible for libvirt
 - Unify name: emulator vs hypervisor
 - Remove print()
 - Avoid running qemu-img when hypervisor is specified as esxi

Note that this commit introduces backward incompatible minion configuration changes from PR #6672, which is yet unreleased. Mentioning it here so that anyone browing that PR will get a heads up on the change.