schema: make pci slot and function optional

schema: make pci slot and function optional

The pcie-switch-downstream-port and pcie-root-port controllers have
only a single slot, numbered 0, and the greate majority of all guest
PCI devices are plugged into function 0 of whatever slot they're
using. The parser makes these optional, setting them to 0 when not
specified, and it's logical for the schema to also make them optional.