image-base.yaml: compress applehv with gzip, hyperv with zip

These platforms are already compressed using those compressors today,
but we're moving that policy out of cosa and into here since it's not a
requirement from the platform itself and more an opinion (which others
may want to differ from).

This uses a new `platform-compressor` key which `cosa compress` will
learn to read.

image-base.yaml

@@ -24,3 +24,9 @@ vmware-os-type: fedora64Guest
 vmware-hw-version: 17
 
 compressor: xz
+
+platform-compressor:
+    # On Windows, it's the only compression tool we can rely on.
+    hyperv: zip
+    # On macOS, it's the only compression tool we can rely on.
+    applehv: gzip