[kernel] Move pml4 create/delete into vm_space

vm_space and page_table continue to take over duties from
page_manager:

- creation and deletion of address spaces / pml4s
- cross-address-space copies for endpoints
- taking over pml4 ownership from process

Also fixed the bug where the wrong process was being set in the cpu
data.

To solve: now the kernel process has its own vm_space which is not
g_kernel_space.

src/kernel/page_table.h

@@ -76,7 +76,7 @@ struct page_table
 		/// Get a *non-const* reference to the current table entry of
 		/// the table at the given level.
 		inline uint64_t & entry(level l) {
-			for (unsigned i = 1; i < unsigned(l); ++i) ensure_table(level(i));
+			for (unsigned i = 1; i <= unsigned(l); ++i) ensure_table(level(i));
 			return table(l)->entries[index(l)];
 		}
 
@@ -149,6 +149,10 @@ struct page_table
 	/// Check if the given entry represents a page (of any size)
 	inline bool is_page(level l, int i) const { return (l == level::pt) || is_large_page(l, i); }
 
+	/// Free this page table and all resources it references
+	/// \arg l  The level of this page table
+	void free(level l);
+
 	/// Print this table to the debug console.
 	void dump(level lvl = level::pml4, bool recurse = true);