virttest.qemu_devices: Support blk_extra_params

blk_extra_params were accidentally left out while rewriting
images creation. Split them and assign them as normal params.

NOTE: This version would work only for params in the standard way
      %s=%s,... Currently I haven't found any other use so I hope
      this will last for ever.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>