Use BIOS characteristics to distinguish EC2 bare-metal from VMs

Use BIOS characteristics to distinguish EC2 bare-metal from VMs

DMI vendor information fields do not provide enough information for us to
distinguish between Amazon EC2 virtual machines and bare-metal instances.
SMBIOS provides a BIOS Information
table (https://www.dmtf.org/sites/default/files/standards/documents/DSP0134_3.4.0.pdf
Ch. 7) that provides a field to indicate that the current machine is a virtual
machine.  On EC2 virtual machine instances, this field is set, while bare-metal
instances leave this unset, so we inspect the field via the kernel's
/sys/firemware/dmi/entries interface.

Fixes #18929