Add provisions for vmnetx-specific metadata in domain XML
Add private xpath helper functions in DomainXML
Allow domain XML to limit the mouse update frequency

To limit mouse events to 40/second, add this to the domain XML's <domain>
element:

<metadata>
  <vx:vmnetx
      xmlns:vx="http://olivearchive.org/xmlns/vmnetx/domain-metadata">
    <vx:limit_mouse_rate hz="40"/>
  </vx:vmnetx>
</metadata>

This is necessary for Windows 3.1, which will exhibit erratic mouse
movement if its input buffer overflows.

Closes #25.