Parallels Desktop 11: verify correct edition and enable headless mode

[rickard-von-essen]

This improves the support for Parallels Desktop for Mac 11 support by:

• Verify you are not trying to build with Standard Edition which lacks necessary command line support.
• Enable headless mode for PD 11 (which was previously ignored).

[rickard-von-essen]

@legal90 could you review this, please.

[legal90]

@rickard-von-essen Thanks!

It looks good to me, but I also think that "headless" mode can be enabled by default for PD 11.
And even more, it could be done just by adding []string{"set", vmName, "--startup-view", "headless"}
to SetDefaultConfiguration func for PD11 structure.
Then we don't need to check driver.SupportHeadless() and s.Headless.

Unlikely Virtualbox, headless mode in Parallels Desktop is more convenient, so user can always get a VM window by clicking VM name in the Parallels Desktop Control Center (VM list in GUI).

[rickard-von-essen]

@legal90 You are correct, I changed the code to always use headless on PD11 and to remove the headless config option for parallels builder. See 83980d2

[legal90]

Great! LGTM

P.s. "Fixer" is a cool thing. We should use it to handle other deprecated options as well (like guest_os_distribution and parallels_tools_host_path)