rust: add IsA bounds to QOM implementation traits

rust: add IsA bounds to QOM implementation traits

Check that the right bounds are provided to the qom_isa! macro
whenever the class is defined to implement a certain class.
This removes the need to add IsA<> bounds together with the
*Impl trait bounds.

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