virt: fix problem of checking persistent in virsh.net_state_dict().

When network is transient, net_autostart() will raise.
The next step will try to get "transient" in result_object.
While error messages may not be in stderr but exist in result
object indeed:

 * Command:
 net-autostart default --disable
 Exit status: 1
 Duration: 0
 stdout:
 error: failed to unmark network default as autostarted
 error: Requested operation is not valid: cannot set autostart
        for transient network

Message "transient" exists, but stderr is None.
So search "ranient" in whole result object will be more suitable.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>