qemu: fix i6300esb watchdog hotplug on Q35

When commit 361c8dc1 added support for hotplugging the i6300esb
watchdog device (first in libvirt-3.9.0), it accidentally contstructed
the commandline for the device_add command before allocating a PCI
address for the device. With no PCI address specified in the command,
the watchdog would simply be placed at the lowest unused PCI slot.

On a 440fx guest, this doesn't cause a problem, because libvirt's PCI
address allocation algorithm would most likely give the same address
anyway (usually a slot on pci-root), so nobody noticed the omission of
address from the command.

But on a Q35 guest, the lowest unused PCI slot is on pcie-root, which
doesn't support hotplug; libvirt knows enough to assign a PCI address
that is on a pcie-to-pci-bridge (because its slots *do* support
hotplug), but qemu doesn't, so if there is no PCI address in the
command, qemu just tries to plug the new device into pcie-root, and
fails because it doesn't support hotplug, e.g.:

  error: Failed to attach device from watchdog.xml
  error: internal error: unable to execute QEMU command 'device_add':
  Bus 'pcie.0' does not support hotplugging

The solution is simply to build the command string after assigning a
PCI address, not before.

Resolves: https://bugzilla.redhat.com/1666559Signed-off-by: NLaine Stump <laine@laine.org>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>

src/qemu/qemu_hotplug.c

@@ -3280,9 +3280,6 @@ qemuDomainAttachWatchdog(virQEMUDriverPtr driver,
     if (qemuAssignDeviceWatchdogAlias(watchdog) < 0)
         return -1;
 
-    if (!(watchdogstr = qemuBuildWatchdogDevStr(vm->def, watchdog, priv->qemuCaps)))
-        return -1;
-
     if (watchdog->model == VIR_DOMAIN_WATCHDOG_MODEL_I6300ESB) {
         if (qemuDomainEnsurePCIAddress(vm, &dev, driver) < 0)
             goto cleanup;
@@ -3294,6 +3291,9 @@ qemuDomainAttachWatchdog(virQEMUDriverPtr driver,
         goto cleanup;
     }
 
+    if (!(watchdogstr = qemuBuildWatchdogDevStr(vm->def, watchdog, priv->qemuCaps)))
+        goto cleanup;
+
     /* QEMU doesn't have a 'dump' action; we tell qemu to 'pause', then
        libvirt listens for the watchdog event, and we perform the dump
        ourselves. so convert 'dump' to 'pause' for the qemu cli */