virtctl: use basename when printing help

[dhiller]

Is this a BUG REPORT or FEATURE REQUEST?:

Uncomment only one, leave it on its own line:

/kind bug

/kind enhancement

What happened:

When used as a krew plugin, kubectl virt [help] prints virtctl instead of kubectl virt

What you expected to happen:

This is how it should be:

$ kubectl virt
Available Commands:
...
  vnc          Open a vnc connection to a virtual machine instance.

Use "kubectl virt <command> --help" for more information about a given command.
Use "kubectl virt options" for a list of global command-line options (applies to all commands).

How to reproduce it (as minimally and precisely as possible):

$ kubectl virt
Available Commands:
...
  vnc          Open a vnc connection to a virtual machine instance.

Use "virtctl <command> --help" for more information about a given command.
Use "virtctl options" for a list of global command-line options (applies to all commands).

Anything else we need to know?:

from krew-index PR

Also a piece of feedback, when you run kubectl virt the help message shows virtctl instead of kubectl virt (you can look at os.args[0] to detect whether running as a plugin or not, the basename would be kubectl-virt[.exe]).

Environment:

• KubeVirt version (use virtctl version): Client Version: version.Info{GitVersion:"v0.17.0", GitCommit:"c0f960702dce718419a767f3913669f539229ff0", GitTreeState:"clean", BuildDate:"2019-05-05T08:09:14Z", GoVersion:"go1.11.5", Compiler:"gc", Platform:"linux/amd64"}
• Kubernetes version (use kubectl version): Client Version: version.Info{Major:"1", Minor:"12", GitVersion:"v1.12.5", GitCommit:"51dd616cdd25d6ee22c83a858773b607328a18ec", GitTreeState:"archive", BuildDate:"2019-03-05T13:36:50Z", GoVersion:"go1.12", Compiler:"gc", Platform:"linux/amd64"}
• VM or VMI specifications: -
• Cloud provider or hardware configuration: -
• OS (e.g. from /etc/os-release): any
• Kernel (e.g. uname -a):
• Install tools:
• Others:

[dhiller]

/assign