Standardise and require safety comments in vmbase.

Bug: 290018030
Test: m vmbase_example_bin
Change-Id: Ic5704d6cd9b2a4090fa9758d7a65a56e83c4286c
diff --git a/vmbase/src/memory/dbm.rs b/vmbase/src/memory/dbm.rs
index d429b30..401022e 100644
--- a/vmbase/src/memory/dbm.rs
+++ b/vmbase/src/memory/dbm.rs
@@ -34,7 +34,7 @@
     } else {
         tcr &= !TCR_EL1_HA_HD_BITS
     };
-    // Safe because it writes to a system register and does not affect Rust.
+    // SAFETY: Changing this bit in TCR doesn't affect Rust's view of memory.
     unsafe { write_sysreg!("tcr_el1", tcr) }
     isb!();
 }