qemu: Use generic PCIe Root Ports by default when available

ioh3420 is emulated Intel hardware, so it always looked
quite out of place in aarch64/virt guests. Even for x86/q35
guests, the recently-introduced pcie-root-port is a better
choice because, unlike ioh3420, it doesn't require IO space
(a fairly constrained resource) to work.

If pcie-root-port is available in QEMU, use it; ioh3420 is
still used as fallback for when pcie-root-port is not
available.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1408808

src/qemu/qemu_domain_address.c

@@ -1846,13 +1846,15 @@ qemuDomainSupportsPCI(virDomainDefPtr def,
 
 
 static void
-qemuDomainPCIControllerSetDefaultModelName(virDomainControllerDefPtr cont)
+qemuDomainPCIControllerSetDefaultModelName(virDomainControllerDefPtr cont,
+                                           virQEMUCapsPtr qemuCaps)
 {
     int *modelName = &cont->opts.pciopts.modelName;
 
     /* make sure it's not already set */
     if (*modelName != VIR_DOMAIN_CONTROLLER_PCI_MODEL_NAME_NONE)
         return;
+
     switch ((virDomainControllerModelPCI)cont->model) {
     case VIR_DOMAIN_CONTROLLER_MODEL_PCI_BRIDGE:
         *modelName = VIR_DOMAIN_CONTROLLER_PCI_MODEL_NAME_PCI_BRIDGE;
@@ -1861,7 +1863,12 @@ qemuDomainPCIControllerSetDefaultModelName(virDomainControllerDefPtr cont)
         *modelName = VIR_DOMAIN_CONTROLLER_PCI_MODEL_NAME_I82801B11_BRIDGE;
         break;
     case VIR_DOMAIN_CONTROLLER_MODEL_PCIE_ROOT_PORT:
-        *modelName = VIR_DOMAIN_CONTROLLER_PCI_MODEL_NAME_IOH3420;
+        /* Use generic PCIe Root Ports if available, falling back to
+         * ioh3420 otherwise */
+        if (virQEMUCapsGet(qemuCaps, QEMU_CAPS_DEVICE_PCIE_ROOT_PORT))
+            *modelName = VIR_DOMAIN_CONTROLLER_PCI_MODEL_NAME_PCIE_ROOT_PORT;
+        else
+            *modelName = VIR_DOMAIN_CONTROLLER_PCI_MODEL_NAME_IOH3420;
         break;
     case VIR_DOMAIN_CONTROLLER_MODEL_PCIE_SWITCH_UPSTREAM_PORT:
         *modelName = VIR_DOMAIN_CONTROLLER_PCI_MODEL_NAME_X3130_UPSTREAM;
@@ -2143,7 +2150,7 @@ qemuDomainAssignPCIAddresses(virDomainDefPtr def,
              * device in qemu) for any controller that doesn't yet
              * have it set.
              */
-            qemuDomainPCIControllerSetDefaultModelName(cont);
+            qemuDomainPCIControllerSetDefaultModelName(cont, qemuCaps);
 
             /* set defaults for any other auto-generated config
              * options for this controller that haven't been