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
cam_periph_releaes_locked() at the end of nvme_probe_start because we
hit an assertion which may be bogus. Instead, leak a periph until we
sort it out. Since these devices don't arrive and depart often, so
this is the lessor of two evils.
MFC after: 1 week
