call mmu_free_roots() to force a cr3 reload. This is awkward because now all the new_cr3 callbacks to the same thing, and because mmu_free_roots() is somewhat of an overkill; but fixing that is more complicated and will be done after this minimal fix. Observed in the Window XP 32-bit installer while bringing up secondary vcpus. Signed-off-by: Avi Kivity 2Tqž)x