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 f1c72795.

Conflicts:

	arch_init.c

Signed-off-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Juan Quintela <quintela@redhat.com>
(cherry picked from commit 9ef051e5)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>