migration: Fix return code of ram_save_iterate()

qemu_savevm_state_iterate() expects the iterators to return 1
when they are done, and 0 if there is still something left to do.
However, ram_save_iterate() does not obey this rule and returns
the number of saved pages instead. This causes a fatal hang with
ppc64 guests when you run QEMU like this (also works with TCG):

 qemu-img create -f qcow2  /tmp/test.qcow2 1M
 qemu-system-ppc64 -nographic -nodefaults -m 256 \
                   -hda /tmp/test.qcow2 -serial mon:stdio

... then switch to the monitor by pressing CTRL-a c and try to
save a snapshot with "savevm test1" for example.

After the first iteration, ram_save_iterate() always returns 0 here,
so that qemu_savevm_state_iterate() hangs in an endless loop and you
can only "kill -9" the QEMU process.
Fix it by using proper return values in ram_save_iterate().

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
