Improve reliability of dynamic loading of vboxj*.jar into classpath.

Forward p.c.vmfest/add-image to p.c.vmfest.service/add-image. This
function used to belong to p.c.vmfest. Also, referencing this function
does not trigger the loading of p.c.vmfest.service, which would break
at compile time since vboxj*.jar gets loaded at runtime when
configuring the first vmfest provider.

Prevent the loading of p.c.vmfest.service if no vboxj*.jar lib is in
the classpath. Will result in exception if you try.