pcie_aer: clear cmask for Advanced Error Interrupt Message Number

The Advanced Error Interrupt Message Number (bits 31:27 of the Root
Error Status Register) is updated when the number of msi messages assigned to a
device changes. Migration of windows 7 on q35 chipset failed because the check
in get_pci_config_device() fails due to cmask being set on these bits. Its valid
to update these bits and we must restore this state across migration.
Signed-off-by: NJason Baron <jbaron@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/pcie_aer.c

@@ -738,6 +738,11 @@ void pcie_aer_root_init(PCIDevice *dev)
                  PCI_ERR_ROOT_CMD_EN_MASK);
     pci_set_long(dev->w1cmask + pos + PCI_ERR_ROOT_STATUS,
                  PCI_ERR_ROOT_STATUS_REPORT_MASK);
+    /* PCI_ERR_ROOT_IRQ is RO but devices change it using a
+     * device-specific method.
+     */
+    pci_set_long(dev->cmask + pos + PCI_ERR_ROOT_STATUS,
+                 ~PCI_ERR_ROOT_IRQ);
 }
 
 void pcie_aer_root_reset(PCIDevice *dev)