virttest.qemu_devices: Support for drive drv_extra_params

Hi guys,

this patch adds the possibility to easily specify custom `-drive xx=yy` params by setting `drv_extra_params` param. It works the same way as existing blk_extra_params.

It's based on the https://github.com/autotest/virt-test/pull/1223 pull request, only the last `281e115` commit is related to this topic.

Kind regards,
Lukáš