pcie_aer: expose pcie_aer_msg() interface

For vfio device, we need to propagate the aer error to
Guest OS. we use the pcie_aer_msg() to send aer error
to guest.

Signed-off-by: Chen Fan <chen.fan.fnst@cn.fujitsu.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
