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>
Merge pull request #1341 from yangdongsheng/snapshot_edit

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