Fix wrong "Failed to list guest" message

The module vmscreenshot is preventing this error using Image.load
function inside a try/except. However right after the except block,
the code calls Image.thumbnail() function, which calls the same
load() function internally. Then the error could happen again.
This patch fixes this possible problem.

Signed-off-by: Rodrigo Trujillo <rodrigo.trujillo@linux.vnet.ibm.com>