QemuOpts: Drop qemu_opt_foreach() parameter abort_on_failure

QemuOpts: Drop qemu_opt_foreach() parameter abort_on_failure

When the argument is non-zero, qemu_opt_foreach() stops on callback
returning non-zero, and returns that value.

When the argument is zero, it doesn't stop, and returns the callback's
value from the last iteration.

The two callers that pass zero could just as well pass one:

* qemu_spice_init()'s callback add_channel() either returns zero or
  exit()s.

* config_write_opts()'s callback config_write_opt() always returns
  zero.

Drop the parameter, and always stop.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>