fixes for vm_analysis

* add pytest.mark.usefixtures("browser" to get screenshots
* fix for stale element exception when first checking if scan option is
available
Merge pull request #502 from dajohnso/fix_vm_analysis

fixes for vm_analysis
disable vm_fleece event_register
Merge pull request #505 from dajohnso/fix_vm_analysis

disable vm_fleece event_register
Change from singledispatch lib to multimethods
* Also update all references to singledispatch
* Fix cfme.web_ui.fill to do proper multidispatch
Convert `utils.error` to use multidispatch
Merge pull request #506 from seandst/multimethods

Multimethods