virttest.qemu_devices: Support for -drive drv_extra_params

In case we want to test some supper hot -drive feature without need
of modifying qemu_devices, we can use drv_extra_params to pass
extra params to -drive. This works the same way blk_extra_params works
for -device extra params.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>