x86/ucode: Support discrete modules being CPIO archives

Multiple downstream distros have tried passing discrete CPIO archives and
tripped over this not working.  It turns out to be easy to support, so do so.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Tested-by: Teddy Astie <teddy.astie@vates.tech>

CHANGELOG.md

@@ -13,6 +13,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
    - Support for Bus Lock Threshold on AMD Zen5 and later CPUs, used by Xen to
      mitigate (by rate-limiting) the system wide impact of an HVM guest
      misusing atomic instructions.
+   - Support for CPIO microcode in discrete multiboot modules.
 
 ### Removed
  - On x86: