On x86-64/q35 and aarch64/virt instances libvirt adds as many
pcie-root-port entries (aka virtual pcie slots) as it needs and adds one
free. If we want to hotplug network interfaces or storage devices then
we quickly run out of available pcie slots.
This patch allows to configure amount of PCIe slots in instance. Method
was discussed with upstream libvirt developers.
To have requested amount of pcie-root-port entries we have to create
whole PCIe structure starting with pcie-root/0 and then add as many
pcie-root-port/0 entries as we want slots. Too low value may get bumped
by libvirt to same as amount of inserted cards.
Systems not using new option will work same way as they did.
Implements: bp configure-amount-of-pcie-ports
Change-Id: Ic3c8761bcde3e842d1b8e1feff1d158630de59ae