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: Jason Baron <jbaron@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
