Configure Nova libvirt.num_pcie_ports to 16 by default

When using the 'q35' machine type, by default, it allows only
a single PCIe device to be hotplugged. Nova currently sets
'num_pcie_ports' to "0" (defaults to libvirt's "1"), which is
not sufficient for hotplug use.

The max value is 28, but 16 is enough in most cases.

More info please check

* [0] https://libvirt.org/pci-hotplug.html
* [1] https://marcin.juszkiewicz.com.pl/2018/02/19/hotplug-in-vm-easy-to-say/
* [2] https://review.opendev.org/c/openstack/nova/+/545034



Co-Authored-By: Bartosz Bezak <bartosz@stackhpc.com>

Change-Id: I7b8063566837aeb947927b7d6858417982b63deb
Closes-Bug: 1909063