hw/core/sysbus: Add a function for creating and attaching an object

hw/core/sysbus: Add a function for creating and attaching an object

A lot of functions are initializing an object and attach it immediately
afterwards to the system bus. Provide a common function for this, which
also uses object_initialize_child() to make sure that the reference
counter is correctly initialized to 1 afterwards.

Reviewed-by: Richard Henderson <[email protected]>
Reviewed-by: Paolo Bonzini <[email protected]>
Reviewed-by: Eduardo Habkost <[email protected]>
Signed-off-by: Thomas Huth <[email protected]>
Reviewed-by: Alistair Francis <[email protected]>
Message-id: 1531745974[email protected]
Signed-off-by: Peter Maydell <[email protected]>