pci: allow 32-bit PCI IO accesses to pass through the PCI bridge

Whilst the underlying PCI bridge implementation supports 32-bit PCI IO
accesses, unfortunately they are truncated at the legacy 64K limit.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
