libvirt_xml: add try/except for remove_by_xpath() failure

del_devices() and remove_all_graphics() maybe suffer from exceptions.
such as, AttributeError and TypeError.
If no element in specified xml file and deleting operation fails,
then use the except block to ignore the error.
Merge pull request #1878 from PandaWei/del_except

libvirt_xml: add try/except for remove_by_xpath() failure