qemu.test: Add enforce_quit test

1. boot guest with -cpu cpu_model,+flag,enforce
2. guest will quit for the following scenerios
 1). cpu_model is advanced than host
 2). +flag isnt support in host
 3). svm is exposed

Change from v1:
1. make info more clear
2. remove blank
3. move force_quit outof exception

Change from v2:
1. change guest_cpu_model
2. get vm after preprocess_vm
3. improve lack_flag

Signed-off-by: Suqin Huang <shuang@redhat.com>