libqos: Solve bug in interrupt checking when using MSIX in virtio-pci.c

The MSIX interrupt was always acked without checking its value, which caused a
race condition. If the ISR was raised between the read and the acking, the ISR
was never detected and it timed out.

Signed-off-by: Marc Marí <marc.mari.barcelo@gmail.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Tested-by: John Snow <jsnow@redhat.com>
Message-id: 1424795655-16952-1-git-send-email-marc.mari.barcelo@gmail.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
