Add snapshot-edit command in virsh.py

1. Add snapshot-edit, snapshot-parent command
2. Modify snapshot-current to output xml/name or command output

Signed-off-by: Kyla Zhang <weizhan@redhat.com>
Change the function place for common use and add options for sync

1. change the function place and improve implementation
2. Add option for sync

Signed-off-by: Kyla Zhang <weizhan@redhat.com>
Add config file for snapshot-edit test

Signed-off-by: Kyla Zhang <weizhan@redhat.com>
add snapshot-edit test

Signed-off-by: Kyla Zhang <weizhan@redhat.com>