ppc/pnv: consolidate pnv_parent_*_fixup() helpers

ppc/pnv: consolidate pnv_parent_*_fixup() helpers

We have 2 helpers that amends the QOM and parent bus of a given object,
repectively. These 2 helpers are called together, and not by accident.
Due to QOM internals, doing an object_unparent() will result in the
device being removed from its parent bus. This means that changing the
QOM parent requires reassigning the parent bus again.

Create a single helper called pnv_parent_fixup(), documenting some of
the QOM specifics that we're dealing with the unparenting/parenting
mechanics, and handle both the QOM and the parent bus assignment.

Next patch will make use of this function to handle a case where we need
to change the QOM parent while keeping the same parent bus assigned
beforehand.

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Frederic Barrat <fbarrat@linux.ibm.com>
Message-Id: <20220819094748.400578-2-danielhb413@gmail.com>