Add vm_set_register() and vm_set_desc() callbacks. These callbacks
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().