vduse: Fix race condition between resetting and irq injecting

The interrupt might be triggered after a reset since there is
no synchronization between resetting and irq injecting. And it
might break something if the interrupt is delayed until a new
round of device initialization.

Fixes: c8a6153b

 ("vduse: Introduce VDUSE - vDPA Device in Userspace")
Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
Link: https://lore.kernel.org/r/20210929083050.88-1-xieyongji@bytedance.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>