python: return dictionary without value in case of no blockjob

python: return dictionary without value in case of no blockjob

Currently, when there is no blockjob, dom.blockJobInfo('vda')
still reports error because it doesn't distinguish return value 0 from -1.
libvirt.libvirtError: virDomainGetBlockJobInfo() failed

virDomainGetBlockJobInfo() API return value:
 -1 in case of failure, 0 when nothing found, 1 found.

And use PyDict_SetItemString instead of PyDict_SetItem when key is
of string type. PyDict_SetItemString increments key/value reference
count, so call Py_DECREF() for value. For key, we don't need to
do this, because PyDict_SetItemString will handle it internally.