Coverity: Fix resource leak in test driver

Coverity: Fix resource leak in test driver

Error: RESOURCE_LEAK:
/builddir/build/BUILD/libvirt-0.9.10/src/test/test_driver.c:1041: alloc_arg: Calling allocation function "virXPathNodeSet" on "devs".
/builddir/build/BUILD/libvirt-0.9.10/src/util/xml.c:621: alloc_arg: "virAllocN" allocates memory that is stored into "*list".
/builddir/build/BUILD/libvirt-0.9.10/src/util/memory.c:129: alloc_fn: Storage is returned from allocation function "calloc".
/builddir/build/BUILD/libvirt-0.9.10/src/util/memory.c:129: var_assign: Assigning: "*((void **)ptrptr)" = "calloc(count, size)".
/builddir/build/BUILD/libvirt-0.9.10/src/util/xml.c:625: noescape: Variable "*list" is not freed or pointed-to in function "memcpy".
/builddir/build/BUILD/libvirt-0.9.10/src/test/test_driver.c:1098: leaked_storage: Variable "devs" going out of scope leaks the storage it points to.