vmm: replace as <pointer> casts with safer alternatives

`as` casts can change mutability, which quickly leads to undefined
behavior.

Signed-off-by: Julian Schindel <mail@arctic-alpaca.de>

vmm/src/device_manager.rs

@@ -4902,7 +4902,7 @@ impl DeviceManager {
                         .remove_userspace_mapping(
                             mapping.addr.raw_value(),
                             mapping.mapping.size(),
-                            mapping.mapping.as_ptr() as _,
+                            mapping.mapping.as_ptr().cast(),
                             mapping.mergeable,
                             mapping.mem_slot,
                         )