Modify the pre_save method on VMStateDescription to return an int
rather than void so that it potentially can fail.

Changed zillions of devices to make them return 0; the only
case I've made it return non-0 is hw/intc/s390_flic_kvm.c that already
had an error_report/return case.

Note: If you add an error exit in your pre_save you must emit
an error_report to say why.
Signed-off-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20170925112917.21340-2-dgilbert@redhat.com>
Reviewed-by: NPeter Xu <peterx@redhat.com>
Reviewed-by: NCornelia Huck <cohuck@redhat.com>
Reviewed-by: NJuan Quintela <quintela@redhat.com>
Signed-off-by: NDr. David Alan Gilbert <dgilbert@redhat.com>