virt: vmgenid: recognize new CID added by Hyper-V

In the Windows spec for VM Generation ID, the originally specified CID
is longer than allowed by the ACPI spec. Hyper-V has added "VMGENCTR" as
a second valid CID that is conformant, while retaining the original CID
for compatibility with Windows guests.

Add this new CID to the list recognized by the driver.

Signed-off-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>