KVM: kvm_vm_ioctl_get_dirty_log restore "nothing dirty" optimization

kvm_vm_ioctl_get_dirty_log scans bitmap to see it it's all zero, but
doesn't use that information.

Avi says:
	Looks like it was used to guard	kvm_mmu_slot_remove_write_access();
	optimizing the case where the guest just leaves the screen alone (which
	it usually does, especially in benchmarks).

	I'd rather reinstate that optimization.  See
	90cb0529 where the damage was done.

It's pretty simple: if the bitmap is all zero, we don't need to do anything to
clean it.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Avi Kivity <avi@qumranet.com>