resume: restore piix pm config registers after resume

On resume, the OS queries the power management event that
caused it. In order to complete this task, it executes some
reads to the piix pm io space. This all happens before the
OS has a chance to restore the PCI config space for devices,
so it is bios's responsibility to make sure the pm IO space
is configured correctly. (During suspend, the piix pm
configuration space is lost).

Note: For 'ordinary' pci devices the config space is
saved by the OS on sleep and restored on resume.

Signed-off-by: Marcel Apfelbaum <marcel.a@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>