libvirt: convert test_blockinfo.py to avoid DB usage

Remove use of the flavour/instance DB access in the blockinfo
test case by using the Instance object instead. This speeds up
execution from approx 3 seconds to 0.3 seconds.

Partial-bug: #1369516
Change-Id: Ibb76ae64edf48012c3eddfe2eb656aa58bd198b1
Merge "libvirt: convert test_blockinfo.py to avoid DB usage"