vm_manager: Make vma_map private

This was only ever public so that code could check whether or not a
handle was valid or not. Instead of exposing the object directly and
allowing external code to potentially mess with the map contents, we
just provide a member function that allows checking whether or not a
handle is valid.

This makes all member variables of the VMManager class private except
for the page table.

src/core/hle/kernel/vm_manager.cpp

@@ -87,6 +87,10 @@ VMManager::VMAHandle VMManager::FindVMA(VAddr target) const {
     }
 }
 
+bool VMManager::IsValidHandle(VMAHandle handle) const {
+    return handle != vma_map.cend();
+}
+
 ResultVal<VMManager::VMAHandle> VMManager::MapMemoryBlock(VAddr target,
                                                           std::shared_ptr<std::vector<u8>> block,
                                                           std::size_t offset, u64 size,