qemu.tests: Add new case check_coredump

    Use guestfs to mount guest image on host mountpoint. And check
    if there is core file in Linux images, bsod in Windows images.

    changelog:
      from V6:
        1. change the past tense to present tense
        2. add timestamp if find the crash directory and file
      from V5:
        1. change the docstring.
        2. change err_msg variable names.
      from V4:
        1. For those image could not be mounted, log the image name.
           And continute to check rest files, do not raise error.
        2. Add more docstring to the functions.
        3. Change the variable names to avoid confusion with method name
      from V3:
        1. remove the redundant slash sign
      from V2:
        1. remove '\n' in the log
        2. change *.qcow2 to *.* to filter all the images
      from V1:
        1. check all the images under the image directory
        2. copy the coredump file to debug dir

Signed-off-by: Xiangmin Han <xhan@redhat.com>