vhost_vdpa: don't setup irq offloading when irq_num < 0

When irq number is negative(e.g., -EINVAL), the virtqueue
may be disabled or the virtqueues are sharing a device irq.
In such case, we should not setup irq offloading for a virtqueue.

Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
Link: https://lore.kernel.org/r/20220222115428.998334-3-lingshan.zhu@intel.com


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