virttest.utils_params: Make object_params thread-safe

Copying of the dictionary is not thread safe. Usually in autotest
we don't need that as object_params are USUALY used for each thread.
This patch makes sure Params.object_params() returns the correct
dictionary.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Merge pull request #1485 from ldoktor/framework

virttest.utils_params: Make object_params thread-safe