RHEV-M avoid failure when accessing instances when at the same time one or more instances is being destroyed.
RHEV-M avoid failure when accessing instances when at the same
time one or more instances is being destroyed.

Fixes: https://issues.apache.org/jira/browse/DTACLOUD-484

lzap/rbovirt,abenari/rbovirt,pparkkin/rbovirt,pdilung/rbovirt