[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/objects/thread.h

@@ -15,7 +15,7 @@ struct TCB
 	uintptr_t rsp;
 	uintptr_t rsp0;
 	uintptr_t rsp3;
-	page_table *pml4;
+	uintptr_t pml4;
 
 	uint8_t priority;
 	// note: 3 bytes padding