virttest.env_process: Add default value to kernel_extra_params related parameters

Add "" as default value for kernel_extra_params_add and kernel_extra_params_remove.
Otherwise it will report TypeError later when we try to add kernel options to them.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Merge pull request #1880 from ypu/msi

virttest.env_process: Add default value to kernel_extra_params related p...