cam: add xpt_path_sim_device to return device_t associated with a path

Return the device associated with the sim's bus when it called
xpt_bus_register, if any. Most real SIMs in the tree set this device,
but some virtual ones do not have a device_t assocaited with them.

Reviewed by: scottl@, mav@ (earlier version)
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D30853