screenshot: fix problem on Power of truncated picture

PIL lib will report error because of picture information inconsistency,
this error may because of qemu screenshot generation,
work around this problem by load this image in advance
to prevent it raise error in thumbnail generation.
REF:
http://stackoverflow.com/questions/12984426/ \
    python-pil-ioerror-image-file-truncated-with-big-images

Signed-off-by: Royce Lv <lvroyce@linux.vnet.ibm.com>