qom: add object_property_add_alias()

qom: add object_property_add_alias()

Sometimes an object needs to present a property which is actually on
another object, or it needs to provide an alias name for an existing
property.

Examples:
  a.foo -> b.foo
  a.old_name -> a.new_name

The new object_property_add_alias() API allows objects to alias a
property on the same object or another object.  The source and target
names can be different.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>