Restore the host's GS.base before returning from 'svm_launch()'.

Previously this was done by the caller of 'svm_launch()' after it returned.
This works fine as long as no code is executed in the interim that depends
on pcpu data.

The dtrace probe 'fbt:vmm:svm_launch:return' broke this assumption because
it calls 'dtrace_probe()' which in turn relies on pcpu data.

Reported by: avg
MFC after: 1 week