virt-qemu: Delete virt_test_type parameter from part of configuration

1.According virttest/bootstrap.py, "virt_test_type" will be added
  automatically in the configuration I modified.
2.steps.cfg belong to "first_subtest" part, "virt_test_type" won't
  be added automatically, it's necessary, so no need to delete this
  parameter from configuration.

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>