hw/pci/aer: Implement PCI_ERR_UNCOR_MASK register

hw/pci/aer: Implement PCI_ERR_UNCOR_MASK register

This register in AER should be both writeable and should
have a default value with a couple of the errors masked
including the Uncorrectable Internal Error used by CXL for
it's error reporting.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Message-Id: <20230302133709.30373-2-Jonathan.Cameron@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Fan Ni <fan.ni@samsung.com>