Add support for up to 16 PCI(e) devices

Add support for up to 16 PCI(e) devices

For non pci express passthrough additional addresses are reserved.
For pcie passthrough pcie root ports are needed (unless guest is like
windows 7).

The first 4 pcie root ports are defined by default in the pve-q35*.cfg
files. If more than 4 pcie devices are passed through the needed root
ports are created on demand. This helps to keep live migration possible
without adding a new pve-q35*.cfg file.

For the windows 7 like guests additional addresses are reserved as well.

Signed-off-by: Aaron Lauterer <a.lauterer@proxmox.com>