pci: switch pci_conf_read32 to use pci_sbdf_t

pci: switch pci_conf_read32 to use pci_sbdf_t

This reduces the number of parameters of the function to two, and
simplifies some of the calling sites.

While there convert {IGD/IOH}_DEV to be a pci_sbdf_t itself instead of
a device number.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Brian Woods <brian.woods@amd.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>