virt: Improve detection of EC2 metal instances

virt: Improve detection of EC2 metal instances

The current detection code relies on /sys/firmware/dmi/entries/0-0/raw
to disambiguate Amazon EC2 virtualized from metal instances.

Unfortunately this file is root only. Thus on a c6g.metal instance
(aarch64), we observe something like this:

$ systemd-detect-virt
amazon
$ sudo systemd-detect-virt
none

Only the latter is correct.

The right long term fix is to extend the kernel to expose the SMBIOS BIOS
Characteristics properly via /sys/class/dmi, but until this happens (and
for backwards compatibility when it does), we need a plan B.

This change implements such a workaround by falling back to using the
instance type from DMI and looking at the ".metal" string present on
metal instances.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>