detect-virt: add bare-metal support for GCE

detect-virt: add bare-metal support for GCE

Google Compute Engine are not only virtual but can be also physical
machines. Therefore checking only the dmi is not enough to detect if it
is a virtual machine. Therefore systemd-detect-virt return "google"
instead of "none" in c3-highcpu-metal machine.
SMBIOS will not help us to make the difference as for EC2 machines.
However, GCE use KVM hypervisor for these VM, we can use this
information to detect virtualization. [0]

Issue and changes has been tested on SUSE SLE-15-SP7 images with
systemd-254 for both GCE, bare-metal and VM.

[0] -
https://cloud.google.com/blog/products/gcp/7-ways-we-harden-our-kvm-hypervisor-at-google-cloud-security-in-plaintext