pc: Add an SMB0 ACPI device to q35

pc: Add an SMB0 ACPI device to q35

This is so I2C devices can be found in the ACPI namespace.  Currently
that's only IPMI, but devices can be easily added now.

Adding the devices required some PCI information, and the bus itself
to be added to the PCMachineState structure.

Note that this only works on Q35, the ACPI for PIIX4 is not capable
of handling an SMBus device.

Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>