qemu.tests.qemu_disk_img: Do not use -c parameter in md5sum command

md5sum in windows guest does not support the parameter -c.

Signed-off-by: Feng Yang <fyang@redhat.com>
qemu.tests.cfg.qemu_disk_img: Update check_files parameter.

Signed-off-by: Feng Yang <fyang@redhat.com>