libvirt: safe_decode domain.XMLDesc(0) for i18n logging

domain.XMLDesc(0) can return a utf-8 encoded string which will cause a
UnicodeDecodeError when substituting the variable in the _LE unicode
translated Message object in oslo.i18n.

This change simply decodes domain.XMLDesc(0) before passing it onto the
logging method when used with a translated message.

Closes-Bug: #1388386

Change-Id: Id56d6564cfb15cc479e664020ae6f1c82acacb09
Merge "libvirt: safe_decode domain.XMLDesc(0) for i18n logging"