translate directly into calls to their namesake API functions in
libvmmapi.
It is an improvement over the existing setreg(), setmsr(), setcr()
setgdt() and exec() callbacks in that the new additions give full
control and don't assume we're booting FreeBSD, like exec() and
don't assume one only wants to set the value of RSP, like setreg().
It uses new API from the part.c to work with partition tables.
Update userboot's disk driver to use new API. Note that struct
loader_callbacks_v1 has changed.
