Added a few vm_object debug functions. Fixed few bugs.

Next issues: For every read fault, the fault must traverse the
vma's object stack until the page is found. The problem was that
we were only searching the first object, that object was a writable
shadow, and the shadow didn't have the read-only page, and the 0
return value was interpreted with IS_ERR() and failed, so address
0 was mapped into the location, and QEMU blew off.

tasks/mm0/src/vm_object.c

@@ -9,6 +9,36 @@
 #include <kmalloc/kmalloc.h>
 
 
+void print_cache_pages(struct vm_object *vmo)
+{
+	struct page *p;
+
+	printf("Pages:\n======\n");
+	list_for_each_entry(p, &vmo->page_cache, list) {
+		printf("Page offset: 0x%x, virtual: 0x%x, refcnt: %d\n", p->offset,
+		       p->virtual, p->refcnt);
+	}
+}
+
+void vm_object_print(struct vm_object *vmo)
+{
+	struct vm_file *f;
+
+	printf("Object type: %s %s. Refs: %d. Pages in cache: %d.\n",
+	       vmo->flags & VM_WRITE ? "writeable" : "read-only",
+	       vmo->flags & VM_OBJ_FILE ? "file" : "shadow", vmo->refcnt,
+	       vmo->npages);
+	if (vmo->flags & VM_OBJ_FILE) {
+		f = vm_object_to_file(vmo);
+		printf("File type: %s\n",
+		       (f->type == VM_FILE_DEVZERO) ? "devzero" :
+		       ((f->type == VM_FILE_BOOTFILE) ? "bootfile" : "regular"));
+		// printf("File type: 0x%x\n", f->type);
+	}
+	print_cache_pages(vmo);
+	printf("\n");
+}
+
 /* Global list of in-memory vm objects. */
 LIST_HEAD(vm_object_list);