tools/libs/guest: Use the system liblz4 in the bzimage loader

Right now lz4, unlike every other compression scheme, unconditionally uses
Xen's unsafe decompressor.  Make it consistent with all other compression
schemes by using liblz4.

The unsafe decompression is still required for the MiniOS build, so rename
xg_dom_decompress_lz4.c to xg_dom_decompress_unsafe_lz4.c and drop the
non-MiniOS content.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>

CHANGELOG.md

@@ -13,6 +13,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
    represent a wildcard input.
  - On x86:
    - Enable pf-fixup option by default for PVH dom0.
+   - The libxenguest bzImage loader now uses the system liblz4 library.
 
 ### Added
  - Support for per-domain Xenstore quota in C xenstored (includes