virttest: Update the kernel command line update part of code in env_process

As the original code for msi setup and extra_kernel_params setup are both
update the guest kernel command line. So remove some duplicate code. And
add the kernel command line restore code in post_process to restore the default
kernel command line in guest. Also add kernel command line check if guest is
started in preprocess.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>