Add parameter `num_pcie_ports` to specify the number of PCIe ports an
instance will get.
Libvirt allows a custom number of PCIe ports (pcie-root-port controllers) a
target instance will get. Some will be used by default, rest will be available
for hotplug use.
Change-Id: I16732c9d6013112381cfad999540dd41ec3d7ba3