Fix compiler warnings in virsh.c

Fix compiler warnings in virsh.c

No functional change. These all generated compiler warnings which, for
some reason weren't converted to errors by
--enable-compiler-warnings=error.

* tools/virsh.c:
  - change return type from int to void on two functions that don't
    return a value.
  - remove unused variables/labels from two functions
  - eliminate non-literal format strings
  - typecast char* into xmlChar* when calling
  - xmlParseBalancedChunkMemory