KVM: Fix dirty bit tracking for slots with large pages

(cherry picked from commit e244584fe3a5c20deddeca246548ac86dbc6e1d1)

When slot is already allocated and being asked to be tracked we need
to break the large pages.

This code flush the mmu when someone ask a slot to start dirty bit
tracking.

Signed-off-by: Izik Eidus <ieidus@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --cc virt/kvm/kvm_main.c
Simple merge