pci_host: Turn into SysBus-derived QOM type

The preceding commits fixed misuses of FROM_SYSBUS() that led people to
add a bogus busdev field. For qdev the field order was less relevant but
for QOM the PCIHostState field (including the SysBusDevice actually
initialized with a value) must be placed first within the state struct.

To facilitate accessing the PCIHostState fields, derive all PCI host
bridges from TYPE_PCI_HOST_BRIDGE rather than TYPE_SYS_BUS_DEVICE.

We can now access PCIHostState QOM-style, with PCI_HOST_BRIDGE() macro.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Signed-off-by: Andreas Färber <andreas.faerber@web.de>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
