autoinstalled paravms should support vnc

[price]

We'd like to have the default setup path create a paravm machine, because that hopefully produces less load and we want most users to use it. To present a convenient interface for a range of different types of users, it'd be good to offer VNC for these machines, as we already do for HVM. This means

• the autoinstall images (for paravm) should have a VNC server set up
• sipb-xen-dev should proxy VNC for these machines as well as the VNC Xen provides for HVMs.