[PATCH] migration: reflect incoming failure to shell

Management apps like libvirt don't know to pay attention to
stderr unless there is a non-zero exit status.

* migration.c (process_incoming_migration_co): Exit with non-zero
status on failure.

Signed-off-by: Eric Blake <address@hidden>
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Noticed while reviewing:
https://lists.gnu.org/archive/html/qemu-devel/2013-04/msg03293.html
and it seems blatant enough to fix now, rather than waiting for
Pavel's series to stabilize.

Side note: libvirt explicitly forbids all use of exit({0,1}), and
instead encourages exit(EXIT_{SUCCESS,FAILURE}), precisely because
it makes it harder to slip in unintentional successful exits.