spapr_pci.c currently has several confusingly similarly named functions for
various conversions between representations of DRCs.  Make things clearer
by renaming things in a more consistent XXX_from_YYY() manner and remove
some called-only-once variants in favour of open coding.

While we're at it, move this code together in the file to avoid some extra
forward references, and split out construction and removal of DRCs for the
host bridge into helper functions.
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>
Acked-by: NMichael S. Tsirkin <mst@redhat.com>