pcie_host: verify mmcfg address range

For a conventional pci device behind
a pcie-to-pci bridge, pci_host handlers get confused by
an out of bounds access in the range [256, 4K).

Check for such an access and make it have no effect.

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
