migration: drop MADVISE_DONT_NEED for incoming zero pages

The madvise for zeroed out pages was introduced when every transferred
zero page was memset to zero and thus allocated. Since commit
211ea740 we check for zeroness of a target page before we memset
it to zero. Additionally we memmap target memory so it is essentially
zero initialized (except for e.g. option roms and bios which are loaded
into target memory although they shouldn't).

It was reported recently that this madvise causes a performance degradation
in some situations. As the madvise should only be called rarely and if it's called
it is likely on a busy page (it was non-zero and changed to zero during migration)
drop it completely.

Reported-By: Zhang Haoyu <haoyu.zhang@huawei.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Juan Quintela <quintela@redhat.com>
