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 V2:
        1. remove '\n' in the log
      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>