lib: Use more of VIR_STEAL_PTR()

We have this very handy macro called VIR_STEAL_PTR() which steals
one pointer into the other and sets the other to NULL. The
following coccinelle patch was used to create this commit:

  @ rule1 @
  identifier a, b;
  @@

  - b = a;
    ...
  - a = NULL;
  + VIR_STEAL_PTR(b, a);

Some places were clean up afterwards to make syntax-check happy
(e.g. some curly braces were removed where the body become a one
liner).

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>

tools/virsh-snapshot.c

@@ -1356,8 +1356,7 @@ virshSnapshotListCollect(vshControl *ctl, virDomainPtr dom,
           virshSnapSorter);
     snaplist->nsnaps -= deleted;
 
-    ret = snaplist;
-    snaplist = NULL;
+    VIR_STEAL_PTR(ret, snaplist);
 
  cleanup:
     virshSnapshotListFree(snaplist);