virsh_snapshot_create_as: Adjustments from review

A review comment/suggestion from @liyangfnst after pull request #1174
was merged to master was to make the entire test use try/finally type
logic so as to only run the xml_recover from one place...

This change implements that concept.
Merge pull request #1199 from jferlan/virsh_snapshot_create_as

virsh_snapshot_create_as: Adjustments from review