virttest.env_process: try to catch windows guest bsod

This patch add bsod images catch when do crash check.
script will check whether the screendump image looks like a
bsod image by calculate the screendump image hamming distance
from the reference image(bsod image). if two images hamming
distance is less than the threshold(10), will mark this
screendump image as suspect, and raise guest BSOD error.
Configuration:
    check_guest_bsod = yes
    bsod_reference_img = XX.jpg (if not configure will use
    all the image under "share/deps/bsod_img")

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