Port to use libxendevicemodel

Port to use libxendevicemodel

This is necessary to allow us to remove functionality from libxenctrl
which is replaced by libxendevicemodel i.e. that which is only used
by QEMU (trad or upstream).

This does everything in one batch for simplicity, it's mostly fairly
straight substitutions.

The goal here (unlike with qemu-upstream) is not necessarily to
abolish all uses of libxenctrl.

Note that when opening the xendevicemodel handle we don't care about
any attempts to restrict, since the full gamut of things needed to
make that effective are only targetting QEMU upstream.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>