libvirt_xml: remove the import of utils_test in devices.

There is a cycle import chain as below:

utils_test->VMXML->librarian->utils_test.

Then we will get a error when we use libvirt_xml:
    from virttest.libvirt_xml import vm_xml
  File "/home/yds/github-autotest/github-autotest/client/tests/virt/virttest/libvirt_xml/__init__.py", line 45, in <module>
    from virttest.libvirt_xml.vm_xml import VMXML
  File "/home/yds/github-autotest/github-autotest/client/tests/virt/virttest/libvirt_xml/vm_xml.py", line 10, in <module>
    from virttest.libvirt_xml.devices import librarian
  File "/home/yds/github-autotest/github-autotest/client/tests/virt/virttest/libvirt_xml/devices/librarian.py", line 6, in <module>
    import virttest.utils_test
  File "/home/yds/github-autotest/github-autotest/client/tests/virt/virttest/utils_test/__init__.py", line 38, in <module>
    import libvirt
  File "/home/yds/github-autotest/github-autotest/client/tests/virt/virttest/utils_test/libvirt.py", line 25, in <module>
    from virttest.libvirt_xml import vm_xml
ImportError: cannot import name vm_xml

This patch remove this unnecessary import line in librarian.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>