Restore imagebackend in test_virt_drivers.py
Restore imagebackend in test_virt_drivers.py

In the test, nova.virt.libvirt.driver.imagebackend is replaced with
fake_imagebackend at setUp(). But is not restored at tearDown(). It
potentially causes failure in other tests expecting imagebackend is
unchanged. Example: test_libvirt.py:test_pre_block_migration_works_correctly.
However it runs without failure since luckily it comes before
test_virt_drivers.py.

Change-Id: I9b61c6c065567472b64bfa363af8f3ba9a502dd2

n0ano/ganttclient