virttest.utils_params: Make object_params thread-safe

Hi guys,

multi_disk test has problems on larger systems (>8 CPUs) that some params are missing. This is due of parallel disk creation, which uses params.object_params() to obtain disk params.

This patch changes the params.object_params() function to be thread-safe.

There is usually not a problem with other params functions as usually each thread in autotest uses different object_params.

Kind regards,
Lukáš