Catch windows bsod using python PIL

This patch tryp to catch windows bsod, if you want using this feature
you should configure following lines in you cfg:
    catch_bsod = yes
    bsod_compare_img = bsod_compare_img.jpg

the bsod_compare_image support absolute and relative path
the default base path is virt
when guest bsod you can see the error log in ERROR log
and the picture place under dir "debug/bsod_dir_$vm_name"

Signed-off-by: Yunping Zheng <yunzheng@redhat.com>