virt: Ignore exception detail if debug is False

For now, when use virsh.domain_exists() or vm.exists()
to check a guest, if it's not exist, there will be some
exception details printed in log, this may confuse users,
this will make them think there is an error happened.

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
Merge pull request #982 from liyangfnst/exist

virt: Ignore exception detail if debug is False