vm: improve the logging when destroying domains

The logging of the failures of the domain destruction
in releaseVm was messy and confusing.
This patch improve the logging and make sure that errors
aren't silently ignored.

Change-Id: Ib314ecd66bb7ccb95da96605d810041f767bc9e0
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28023
Reviewed-by: Dan Kenigsberg <danken@redhat.com>