restore: use less convoluted method to calculate the number of zombies

Instead of doing this crazy thing where we pass the number of zombies back
from the restorer blob to the main restore thread, let's just calculate the
number in the main restore thread when we need it.

Also, note that this patch shifts the location of this calculation: we used
to do a decrement of nr_in_progress when we waited() on a zombie, to sort
of lie about the fact that they did go through CR_STATE_RESTORE_SIGCHLD,
but since we're waiting on them anyway explicitly, there's no reason to do
all this hackery. Let's just calculate the number once in the right place
and be done with it.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>