string-table: Introduce DECLARE_STRING_TABLE_LOOKUP() and friends

Let's introduce these for the following two reasons:
- Using them makes sure the parameter names match between declaration
  and definition (if both use the corresponding macros).
- They make sure developers can't forget the _const_ and _pure_
  attributes for the declarations.

This commit also includes some include sorting fixes

src/basic/virt.h

@@ -62,6 +62,5 @@ Virtualization detect_virtualization(void);
 int running_in_userns(void);
 int running_in_chroot(void);
 
-const char* virtualization_to_string(Virtualization v) _const_;
-Virtualization virtualization_from_string(const char *s) _pure_;
+DECLARE_STRING_TABLE_LOOKUP(virtualization, Virtualization);
 bool has_cpu_with_flag(const char *flag);