libvirt: improve unit test time

There were unit tests that were taking more than 4 seconds.

nova.tests.virt.libvirt.test_driver.LibvirtConnTestCase.test_clean_shutdown_failure    5.091
nova.tests.virt.libvirt.test_driver.LibvirtConnTestCase.test_clean_shutdown_with_retry 4.093

TrivialFix

Change-Id: I75cdb74dca69a1e31d6b974148c9590328c362e1
Closes-bug: #1366549
Merge "libvirt: improve unit test time"