Add a virGetLastErrorMessage() function

Add a virGetLastErrorMessage() function

Apps using libvirt will often have code like

   if (virXXXX() < 0) {
      virErrorPtr err = virGetLastError();
      fprintf(stderr, "Something failed: %s\n",
              err && err->message ? err->message :
              "unknown error");
      return -1;
   }

Checking for a NULL error object or message leads to very
verbose code. A virGetLastErrorMessage() helper from libvirt
can simplify this to

   if (virXXXX() < 0) {
      fprintf(stderr, "Something failed: %s\n",
              virGetLastErrorMessage());
      return -1;
   }

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>