Switch to linker sets to find the xport callback object. This

Switch to linker sets to find the xport callback object.  This
eliminates the need to special case everything in cam_xpt for new
transports. It is now a failure to not have a transport object when
registering the bus as well. You can still, however, create a
transport that's unspecified (XPT_)

Differential Revision: https://reviews.freebsd.org/D7289