PATCH v2: Implement NFS Protocol

[Ryan Gahagan]

Per Issue 90[1], QEMU supports using an NFS disk but Libvirt had no
support for attaching a disk of this type. This patch series adds in
this capability as well as the tests necessary to make sure it works
correctly.

The changes in this second version over the previous patch series for
this issue primarily address feedback. Documentation was updated and the
XML <nfs> element has been renamed as an <identity> element. The code
for checking user XML for an NFS protocol has been relocated to the
validation method and other pieces of code were refactored per
suggestion. Tests have been altered to reflect the new XML element and
the fact that NFS does not allow hosts to have a port.

References:
[1]: https://gitlab.com/libvirt/libvirt/-/issues/90