virt: Convert cpu_baseline ElementTree to string
virt: Convert cpu_baseline ElementTree to string

In commit 0f5184c (Remove minidom use in virt module) the value
of `cpu` become `xml.etree.ElementTree.Element` and no longer
has a method `toxml()`. This results in the following error:

$ salt '*' virt.cpu_baseline
host2:
    The minion function caused an exception: Traceback (most recent call last):
      File "/usr/lib/python3.7/site-packages/salt/minion.py", line 1675, in _thread_return
        return_data = minion_instance.executors[fname](opts, data, func, args, kwargs)
      File "/usr/lib/python3.7/site-packages/salt/executors/direct_call.py", line 12, in execute
        return func(*args, **kwargs)
      File "/usr/lib/python3.7/site-packages/salt/modules/virt.py", line 4410, in cpu_baseline
        return cpu.toxml()
    AttributeError: 'xml.etree.ElementTree.Element' object has no attribute 'toxml'

Signed-off-by: Radostin Stoyanov <af191057dab5b67c1c1061d181b58b884c4ed7f3@fedoraproject.org>

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt