Standardize logging for nova.virt.libvirt

 - add log translation hints for warning, error and info levels
 - remove use of % as a string formatter, use the log functionality
   instead
 - move from LOG.warning to LOG.warn

Log translation is motivated by oslo's move to prioritized
translation of strings, as documented at
https://wiki.openstack.org/wiki/LoggingStandards#Log_Translation

Change-Id: I61ab8dbcdb73795538ea9d60c0c9d00c425ee1a5
Merge "Standardize logging for nova.virt.libvirt"