pseries: Use correct dispatcher for PCI config space accesses

The pseries machine expects a para-virtualized guest and so supplies RTAS
functions (via a hypercall) for performing PCI config space access.
Currently the implementation of these calls into
pci_default_{read,write}_config().  However this would be incorrect for
any PCI device which overrides the default config read/write functions.
AFAICT there's only one such device today, but we should still get it
right.  In addition the pci_host_config_{read,write}_common() functions
which do correctly do this dispatch, perform bounds checking on the config
space address, lack of which currently leads to an exploitable bug.

This patch corrects the problem.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>
