test.iofuzz: some modifications of iofuzz

This patch make some modifications on iofuzz:
1. When session is not response, check whether qemu or guest kernel
   crash occurs.
2. Check whether qemu or guest kernel crash occurs after fuzz test.
3. Make the script follow pep8 rules
4. enable image snapshot.

Signed-off-by: Yunping Zheng <yunzheng@redhat.com>