make global_t instances static

These structures are mainly used for purpose of easier debugging.
That means - instead of adding each member separately "on the watch"
of the debugger, the whole structure is put on the watch.

Also, these structures shouldn't be accessible outside of the compilation
unit, therefore it makes sense to make them static. This has also benefit
of avoiding potential name conflicts.

src/hvpp/hvpp/hypervisor.cpp

@@ -64,7 +64,7 @@ namespace hvpp::hypervisor
     bool    running;
   };
 
-  global_t global;
+  static global_t global;
 
   auto start(vmexit_handler& handler) noexcept -> error_code_t
   {

src/hvpp/hvpp/lib/mm.cpp

@@ -18,7 +18,7 @@ namespace mm
     object_t<mtrr_descriptor_t> mtrr_descriptor;
   };
 
-  global_t global;
+  static global_t global;
 
   allocator_guard::allocator_guard() noexcept
     : allocator_guard(global.custom_allocator)