conf: Prefer pcie-to-pci-bridge to dmi-to-pci-bridge

Both pcie-to-pci-bridge and dmi-to-pci-bridge can be used to
create a traditional PCI topology in a pure PCIe guest such as
those using the x86_64/q35 or aarch64/virt machine type;
however, the former should be preferred, as it doesn't need to
obey limitation of real hardware and is completely
architecture-agnostic.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1520821

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>

docs/formatdomain.html.in

@@ -4114,10 +4114,14 @@
       devices (although libvirt will never auto-assign a PCI device to
       a PCIe slot, it will allow manual specification of such an
       assignment). Devices connected to pcie-root cannot be
-      hotplugged. In order to make standard PCI slots available on a
-      system which has a pcie-root controller, a pci controller
-      with <code>model='dmi-to-pci-bridge'</code> is automatically
-      added, usually at the defacto standard location of slot=0x1e. A
+      hotplugged. If traditional PCI devices are present in the guest
+      configuration, a <code>pcie-to-pci-bridge</code> controller will
+      automatically be added: this controller, which plugs into a
+      <code>pcie-root-port</code>, provides 31 usable PCI slots (1-31) with
+      hotplug support (<span class="since">since 4.3.0</span>). If the QEMU
+      binary doesn't support the corresponding device, then a
+      <code>dmi-to-pci-bridge</code> controller will be added instead,
+      usually at the defacto standard location of slot=0x1e. A
       dmi-to-pci-bridge controller plugs into a PCIe slot (as provided
       by pcie-root), and itself provides 31 standard PCI slots (which
       also do not support device hotplug). In order to have