xen-hvm: stop faking I/O to access PCI config space

This patch removes the current hackery where IOREQ_TYPE_PCI_CONFIG
requests are handled by faking PIO to 0xcf8 and 0xcfc and replaces it
with direct calls to pci_host_config_read/write_common().
Doing so necessitates mapping BDFs to PCIDevices but maintaining a simple
QLIST in xen_device_realize/unrealize() will suffice.

NOTE: whilst config space accesses are currently limited to
      PCI_CONFIG_SPACE_SIZE, this patch paves the way to increasing the
      limit to PCIE_CONFIG_SPACE_SIZE when Xen gains the ability to
      emulate MCFG table accesses.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>