KVM: MMU: awareness of new kvm_mmu_zap_page behaviour

kvm_mmu_zap_page will soon zap the unsynced children of a page. Restart
list walk in such case.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>