test: Implement fake storage pool driver in qemuxml2argv test

test: Implement fake storage pool driver in qemuxml2argv test

To support testing of "volume" disk backing, we need to implement a few
disk driver backend functions.

The fake storage driver uses files in storagepoolxml2xmlout/POOLNAME.xml
as XML files for pool definitions and volume names are in format
"VOL_TYPE+VOL_PATH". By default type "block" is assumed (for iSCSI test
compatibility).

The choice of this approach along with implemented functions was made so
that <disk type='volume'> can be tested in the xml2argv test.