kvmclock: Add comment explaining why we need cpu_clean_all_dirty()

kvmclock: Add comment explaining why we need cpu_clean_all_dirty()

Try to explain why commit 317b0a6d8ba44e9bf8f9c3dbd776c4536843d82c
needed a cpu_clean_all_dirty() call just after calling
cpu_synchronize_all_states().

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Cc: Andrey Korolyov <andrey@xdel.ru>
Cc: Marcin Gibuła <m.gibula@beyond.pl>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit 1154d84dcc5f46e83db94281d071775819dd8884)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>