Ensure name is utf-8 when deleting rbd vol or snap

If the name supplied to librbd is not utf-8 e.g. unicode,
an exception is raised. This was previously fixed and
subsequently broken by commit cbe1d5f.

Change-Id: I29f5e9db87ceb5f9cfe3be43e76d3e860edaf6d7
Closes-Bug: bug 1292433
Merge "Ensure name is utf-8 when deleting rbd vol or snap"