Revert "migration: do not sent zero pages in bulk stage"

Not sending zero pages breaks migration if a page is zero
at the source but not at the destination. This can e.g. happen
if different BIOS versions are used at source and destination.
It has also been reported that migration on pseries is completely
broken with this patch.

This effectively reverts commit f1c72795af573b24a7da5eb52375c9aba8a37972.

Conflicts:

	arch_init.c
	qmp-commands.hx

Signed-off-by: Peter Lieven <pl@kamp.de>
migration: do not overwrite zero pages

on incoming migration do not memset pages to zero if they already read as zero.
this will allocate a new zero page and consume memory unnecessarily. even
if we madvise a MADV_DONTNEED later this will only deallocate the memory
asynchronously.

Signed-off-by: Peter Lieven <pl@kamp.de>