Virtualbox fixes

This is gh-823 and gh-913 rebased onto develop.

----

The "RELEASE" target type has been untested for sometime. Instead the "QA"
type has always been built and then renamed on release. For the moment, we'll
simplify the configuration and only use "QA" builds.

Since the support for primitive result sets with the xpath query parameter in
the Jenkins REST API has been dropped, text() cannot be used anymore. Instead
we parse the XML block read from the API for the latest successful build URL
and server file.

----

Note: [docs/installVM/setup_omero.sh](docs/installVM/setup_omero.sh) is now be the same on dev_4_4 and develop with `OMERO_JOB` set to `OMERO-stable` by default (assuming it is the reponsibility of the Jenkins job to pass `OMERO-trunk`.