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 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>