Resolve Coverity complaint in storagevolxml2argvtest

Resolve Coverity complaint in storagevolxml2argvtest

Ignore NULL pool in testSetVolumeType to silence Coverity,
even though we only call it with NULL pool when vol is also NULL.

(13) Event var_deref_model:	Passing null pointer "inputpool" to
function "testSetVolumeType(virStorageVolDefPtr, virStoragePoolDefPtr)",
which dereferences it. [details]
Also see events: [assign_zero]

95    testSetVolumeType(inputvol, inputpool);