Only try to retrieve UUID from SMBIOS on x86 machines

[lwhsu]

Summary

This is a new version of D9384, updated with jhb's suggestion and patch against
the new location of hostid file.

Proposed commit message:

Only try to retrieve UUID from SMBIOS on x86 machines

This prevents an useless warning message on non-x86 machines.

Also rename hostid_hardware() to hostid_smbios() to reflect what it actually
does.  In the future we may introduce more hostid_foo functions to obtain UUID
from firmware on other platforms.

Original work by: linimon

[val_packett.cool]

Server-class arm64 machines also have SMBIOS data present. Maybe just condition the actual warning print to amd64 | i386 but try it everywhere?