Fixing races for libvirt connection

The code had multiple cases in which the libvirt connection
was used by a thread when another thread could close it. This
was a rare occurence, but it has been seen. The intention is
to avoid such races by moving from check_hypervisor_conn()
function in favor of paired invocations as follows:

   conn = lock_hypervisor_conn()
   ...
   unlock_hypervisor_conn()

Fixes EUCA-5337
Merge branch 'dev/dmitrii/EUCA-5337' into testing
tweaks to log messages