aq(4) attached only with MSI-X. Without an MSI controller iflib falls
back to INTx, and attach failed with EOPNOTSUPP.
Accept IFLIB_INTR_LEGACY, program the controller for INTx, and add a
legacy filter. INTx is level-triggered and nothing auto-masks, so the
filter masks the vector before acknowledging it; iflib unmasks it via
IFDI_INTR_ENABLE once the queues are drained. All causes already map
to vector 0 in this mode. MSI stays unsupported: iflib never selects
it here, since these controllers advertise 32 messages.