conf: Fix memory leaks in virStoragePoolDefParseSource

conf: Fix memory leaks in virStoragePoolDefParseSource

Detected by valgrind. Leaks are introduced in commit 122fa379.

src/conf/storage_conf.c: fix memory leaks.

How to reproduce?
$ make && make -C tests check TESTS=storagepoolxml2xmltest
$ cd tests && valgrind -v --leak-check=full ./storagepoolxml2xmltest

actual result:
==28571== LEAK SUMMARY:
==28571==    definitely lost: 40 bytes in 5 blocks
==28571==    indirectly lost: 0 bytes in 0 blocks
==28571==      possibly lost: 0 bytes in 0 blocks
==28571==    still reachable: 1,054 bytes in 21 blocks
==28571==         suppressed: 0 bytes in 0 blocks

Signed-off-by: Alex Jia <ajia@redhat.com>