piix: create host bridge to passthrough

Implement a pci host bridge specific to passthrough. Actually
this just inherits the standard one. And we also just expose
a minimal real host bridge pci configuration subset.

[Replace pread with lseek and read to fix Windows build]

Signed-off-by: Tiejun Chen <tiejun.chen@intel.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
