libvirt: use handle_prompts() to wait for end of 'ZZ' in session.

Currently, we use time.sleep(1) to wait for the end of 'ZZ' in session,
but it does not work well in any conditions.

This patch replace time.sleep(1) with remote.handle_prompts().

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>