Remove Support for VixDiskLib 5.1.0

VixDiskLib 5.1.0 has a known bug intermittently causing a
segfault during disk open operations when the vCenter Server
does not provide an SSL thumbprint for the target host.  The
code has been modified to allow unsupported versions to be
specified in the "bad_versions" array.

@roliveri @jrafanie @chessbyte please review.
Comments can quickly become stale, so if it's useful to keep track of why they're "bad", maybe the comment can be used in the error message directly, or even link to the vddk release notes.  Just a thought.

```ruby
bad_versions = { 5.1.0 => "Disk Open May Core Dump without an SSL Thumbprint"}
...
if bad_versions.keys.include?(version)
  loaded_library = ""
  raise LoadError, "VixDiskLib #{version} is not supported: #{bad_versions[version]}"
end
...
```