Create xpt_sim_poll and refactor a bit using it.

xpt_sim_poll takes the sim to poll as an argument. It will do the
proper locking protocol, call the SIM polling routine, and then call
camisr_runqueue to process completions on any CCBs the SIM's poll
routine completed. It will be used during late shutdown when a SIM is
waiting for CCBs it sent during shutdown to finish and the scheduler
isn't running because we've panic'd.

This sequence was used twice in cam_xpt, so refactor those to use this
new function.

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