rust: qom: add casting functionality

rust: qom: add casting functionality

Add traits that let client cast typecast safely between object types.
In particular, an upcast is compile-time guaranteed to succeed, and a
YOLO C-style downcast must be marked as unsafe.

The traits are based on an IsA<> trait that declares what
is a subclass of what, which is an idea taken from glib-rs
(https://docs.rs/glib/latest/glib/object/trait.IsA.html).
The four primitives are also taken from there
(https://docs.rs/glib/latest/glib/object/trait.Cast.html).  However,
the implementation of casting itself is a bit different and uses the
Deref trait.

This removes some pointer arithmetic from the pl011 device; it is also a
prerequisite for the definition of methods, so that they can be invoked
on all subclass structs.  This will use the IsA<> trait to detect the
structs that support the methods.

glib also has a "monadic" casting trait which could be implemented on
Option (as in https://docs.rs/glib/latest/glib/object/trait.CastNone.html)
and perhaps even Result.  For now I'm leaving it out, as the patch is
already big enough and the benefit seems debatable.

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>