QemuOpts: Drop qemu_opts_foreach() parameter abort_on_failure

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

When the argument is zero, it doesn't stop, and returns the bit-wise
inclusive or of all the return values.  Funky :)

The callers that pass zero could just as well pass one, because their
callbacks can't return anything but zero:

* qemu_add_globals()'s callback qdev_add_one_global()

* qemu_config_write()'s callback config_write_opts()

* main()'s callbacks default_driver_check(), drive_enable_snapshot(),
  vnc_init_func()

Drop the parameter, and always stop.

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