Previously libvirt would only add pci-bridge devices automatically
when an address was requested for a device that required a legacy PCI
slot and none was available. This patch expands that support to
dmi-to-pci-bridge (which is needed in order to add a pci-bridge on a
machine with a pcie-root), and pcie-root-port (which is needed to add
a hotpluggable PCIe device). It does *not* automatically add
pcie-switch-upstream-ports or pcie-switch-downstream-ports (and
currently there are no plans for that).

Given the existing code to auto-add pci-bridge devices, automatically
adding pcie-root-ports is fairly straightforward. The
dmi-to-pci-bridge support is a bit tricky though, for a few reasons:

1) Although the only reason to add a dmi-to-pci-bridge is so that
   there is a reasonable place to plug in a pci-bridge controller,
   most of the time it's not the presence of a pci-bridge *in the
   config* that triggers the requirement to add a dmi-to-pci-bridge.
   Rather, it is the presence of a legacy-PCI device in the config,
   which triggers auto-add of a pci-bridge, which triggers auto-add of
   a dmi-to-pci-bridge (this is handled in
   virDomainPCIAddressSetGrow() - if there's a request to add a
   pci-bridge we'll check if there is a suitable bus to plug it into;
   if not, we first add a dmi-to-pci-bridge).

2) Once there is already a single dmi-to-pci-bridge on the system,
   there won't be a need for any more, even if it's full, as long as
   there is a pci-bridge with an open slot - you can also plug
   pci-bridges into existing pci-bridges. So we have to make sure we
   don't add a dmi-to-pci-bridge unless there aren't any
   dmi-to-pci-bridges *or* any pci-bridges.

3) Although it is strongly discouraged, it is legal for a pci-bridge
   to be directly plugged into pcie-root, and we don't want to
   auto-add a dmi-to-pci-bridge if there is already a pci-bridge
   that's been forced directly into pcie-root.

Although libvirt will now automatically create a dmi-to-pci-bridge
when it's needed, the code still remains for now that forces a
dmi-to-pci-bridge on all domains with pcie-root (in
qemuDomainDefAddDefaultDevices()). That will be removed in a future
patch.

For now, the pcie-root-ports are added one to a slot, which is a bit
wasteful and means it will fail after 31 total PCIe devices (30 if
there are also some PCI devices), but helps keep the changeset down
for this patch. A future patch will have 8 pcie-root-ports sharing the
functions on a single slot.