libstore: Add apple-virt to system features when available

I'm sure that we'll adjust the implementation over time, but this
at least discerns between an apple silicon bare metal machine and
a tart VM.

src/libstore/globals.hh

@@ -714,6 +714,10 @@ public:
 
           System features are user-defined, but Nix sets the following defaults:
 
+          - `apple-virt`
+
+            Included on darwin if virtualization is available.
+
           - `kvm`
 
             Included by default if `/dev/kvm` is accessible.