cam: add xpt_path_device to return device_t associated with a path

[imp]

Summary

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.

Sponsored by: Netflix

[mav]

I've just thought that struct cam_path has own device field. So the function name is confusing.

[imp]

In D30853#694698, @mav wrote:

I've just thought that struct cam_path has own device field. So the function name is confusing.

I'll change to xpt_path_sim_device().