Fix parameters for LibvirtXMLError calls

Initially seen in vm_xml/sync() - the raising of the exception passed
3 paramters instead of the __init__ required 2:

   File "/home/virt-test/virttest/libvirt_xml/vm_xml.py", line 376, in sync
     raise xcepts.LibvirtXMLError("Failed to undefine %s.", self.vm_name)
 TypeError: __init__() takes at most 2 arguments (3 given)

Since I was fixing this one - I looked for others. Found the pool_xml/
pool_rename() condition as well, so I modified it too.
Merge pull request #1447 from jferlan/except_fix

Fix parameters for LibvirtXMLError calls