conf: auto-add a pcie-root-port when needed while plugging in pcie-to-pci-bridge

conf: auto-add a pcie-root-port when needed while plugging in pcie-to-pci-bridge

This will almost surely never come up during any normal operation[*],
which is likely why this wasn't done when pcie-to-pci-bridge support
was added back in the before-fore times. It's pretty simple to support
though - a pcie-to-pci-bridge plugs into a pcie-root-port just like
any other pcie device, and if there isn't an open slot on an existing
pcie-root-port, we can just add one.

([*] in real life, a pcie-to-pci-bridge is only auto-added by libvirt
itself, while this function is dealing with the followup to *user
added* devices. Also each pcie-to-pci-bridge has 32 slots, and it's
unlikely a domain with pcie support would be wanting more than 32
conventional PCI (i.e. not PCIe) devices)

Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>