commit 62c96360
    virtio-pci: fix level interrupts
only helps systems without irqfd: on systems with irqfd support we
passed in flag requesting irqfd even when msix is disabled.

As a result, for level interrupts we didn't install an fd handler so
unmasking an fd had no effect.

Fix this up.

Cc: qemu-stable@nongnu.org
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>