pci/aer: fix error injection

Fix the injection logic upon aer message to follow 6.2.4.1.2 more
closely: specifically only send an msi interrupt when the logical or of
the enabled bits changed, not when a bit which was previously clear
becomes set.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
