Call xpt_sim_poll in shutdown_final handler.

When we're shutting down, we send a number of start/stop commands to
the known targets. We have to wait for them to complete. During a
panic, the interrupts are off, and using pause to wait for them to
fire and complete won't work: we have to poll after pause returns so
the completion routines of the CCBs run so we decrement work
outstanding counts.

Sponsored by: Netflix
Differential Review: https://reviews.freebsd.org/D16663