migration: Fix madvise breakage if host and guest have different page sizes

madvise(DONTNEED) will throw away the contents of the whole page at the
given address, even if the given length is less than the page size.  One
can argue about whether that's the correct behaviour, but that's what it's
done for a long time in Linux at least.

That means that the madvise() in ram_load(), on a setup where
TARGET_PAGE_SIZE is smaller than the host page size, can throw away data
in guest pages adjacent to the one it's actually processing right now,
leading to guest memory corruption on an incoming migration.

This patch therefore, disables the madvise() if the host page size is
larger than TARGET_PAGE_SIZE.  This means we don't get the benefits of that
madvise() in this case, but a more complete fix is more difficult to
accomplish.  This at least fixes the guest memory corruption.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reported-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
