vfio/pci: Call vfio_prepare_kvm_msi_virq_batch() in MSI retry path

When vfio_enable_vectors() returns with less than requested nr_vectors
we retry with what kernel reported back. But the retry path doesn't
call vfio_prepare_kvm_msi_virq_batch() and this results in,

qemu-system-aarch64: vfio: Error: Failed to enable 4 MSI vectors, retry with 1
qemu-system-aarch64: ../hw/vfio/pci.c:602: vfio_commit_kvm_msi_virq_batch: Assertion `vdev->defer_kvm_irq_routing' failed

Fixes: dc580d5 ("vfio: defer to commit kvm irq routing when enable msi/msix")
Reviewed-by: Longpeng <longpeng2@huawei.com>
Signed-off-by: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>
(cherry picked from commit c174088)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

hw/vfio/pci.c

@@ -663,15 +663,15 @@ static void vfio_msi_enable(VFIOPCIDevice *vdev)
 
     vfio_disable_interrupts(vdev);
 
+    vdev->nr_vectors = msi_nr_vectors_allocated(&vdev->pdev);
+retry:
     /*
      * Setting vector notifiers needs to enable route for each vector.
      * Deferring to commit the KVM routes once rather than per vector
      * provides a substantial performance improvement.
      */
     vfio_prepare_kvm_msi_virq_batch(vdev);
 
-    vdev->nr_vectors = msi_nr_vectors_allocated(&vdev->pdev);
-retry:
     vdev->msi_vectors = g_new0(VFIOMSIVector, vdev->nr_vectors);
 
     for (i = 0; i < vdev->nr_vectors; i++) {