Drivers: hv: vmbus: Deactivate sysctl_record_panic_msg by default in isolated guests

In the Linux kernel, the following vulnerability has been resolved: Drivers: hv: vmbus: Deactivate sysctl_record_panic_msg by default in isolated guests hv_panic_page might contain guest-sensitive information, do not dump it over to Hyper-V by default in isolated guests. While at it, update some comments in hyperv_{panic,die}_event().