kvm: update_memslots: clean flags for invalid memslots

Indeed, any invalid memslots should be new->npages = 0,
new->base_gfn = 0 and new->flags = 0 at the same time.

Signed-off-by: Tiejun Chen <tiejun.chen@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>