virttest: Add unittest for libvirt_storage module.

* Replace real virsh module with a bogus virsh module
* Rewrite relative virsh pool functions for unittest.
  With setting output to static strings.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
virt-libvirt: Fix small bug in storage_unittest

The pool_autostart function was not being overriden in virsh instance,
but dummy function already written.  I just added it into setUp() now
unittest is working perfectly when run as non-root.

Signed-off-by: Chris Evich <cevich@redhat.com>