Free slicename in virSystemdCreateMachine

https://bugzilla.redhat.com/show_bug.cgi?id=1008619

1,003 bytes in 1 blocks are definitely lost in loss record 599 of 635
==404== by 0x50728A7: virBufferAddChar (virbuffer.c:185)
==404== by 0x50BC466: virSystemdEscapeName (virsystemd.c:67)
==404== by 0x50BC6B2: virSystemdMakeSliceName (virsystemd.c:108)
==404== by 0x50BC870: virSystemdCreateMachine (virsystemd.c:169)
==404== by 0x5078267: virCgroupNewMachine (vircgroup.c:1498)
(cherry picked from commit 09b48562)

src/util/virsystemd.c

@@ -244,5 +244,6 @@ cleanup:
     VIR_FREE(username);
     VIR_FREE(creatorname);
     VIR_FREE(machinename);
+    VIR_FREE(slicename);
     return ret;
 }