virtio_pci: fix level interrupts with irqfd

commit 62c96360ae7f2c7a8b029277fbb7cb082fdef7fd
    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.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>