cc-check should only warn if "unrestricted_guest" is not set when running under a hypervisor

[jodh-intel]

Now that virtcontainers provides a way to detect if running under a hypervisor...

• qemu: Detect VMM environment to apply specific flags to our VM containers/virtcontainers#343

... we need to update cc-check to determine more intelligently if the environment is capable of running a Clear Container since although environments like Azure do not provide unrestricted_guest, it is still possible to run a Clear Container on it.

In summary, cc-check needs to do the following:

• if running under a hypervisor, only warn if unrestricted_guest is not set (since it does work under Azure for example).
• If not running under a hypervisor, error if unrestricted_guest is not set (no change to current behaviour).

See comments on #422.

[sboeuf]

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