virttest.qemu_devices: Fix the error caused by empty bus_extra_params

If bus_extra_params is empty string, it will raise error in the following
code. So update the code to fix it.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Merge pull request #1889 from ypu/bus_extra_params

virttest.qemu_devices: Fix the error caused by empty bus_extra_params