Fix VM snapshot handling for combined INBOX.

This should make it more clear about what's happening.

Review: https://reviewboard.asterisk.org/r/2539/
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Merged revisions 388816 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@388818 f38db490-d61c-443f-a65b-d21fe96a405b