virt: detect Amazon EC2 Nitro instance

virt: detect Amazon EC2 Nitro instance

Amazon EC2 Nitro hypervisor is technically based on KVM[1], which
systemd-detect-virt identify propely from CPUID. However the lack of
CPUID on aarch64 (A1, T4 instance type) prevents a correct
identification, impacting hostnamectl and systemd-random-seed. Instead
it's possible to identify virtualization from DMI vendor ID.

Prior to this commit:
  # hostnamectl
     Static hostname: n/a
  Transient hostname: ip-10-97-8-12
           Icon name: computer
          Machine ID: 8e3772fbcfa3dd6f330a12ff5df5a63b
             Boot ID: b7b7e2fe0079448db664839df59f9817
    Operating System: Gentoo/Linux
              Kernel: Linux 5.4.69-longterm
        Architecture: arm64

After this commit:
  # hostnamectl
     Static hostname: n/a
  Transient hostname: ip-10-97-8-12
           Icon name: computer-vm
             Chassis: vm
          Machine ID: 8e3772fbcfa3dd6f330a12ff5df5a63b
             Boot ID: bd04da57084e41078f20541101867113
      Virtualization: amazon
    Operating System: Gentoo/Linux
              Kernel: Linux 5.4.69-longterm
        Architecture: arm64

[1] https://aws.amazon.com/ec2/faqs/