xenlight: enable emulated NICs (as well as PV)

libxl wants us to say NIC_TYPE_VIF_IOEMU to request both a VIF and
an emulated interface, rather than only NIC_TYPE_VIF.

Simplify the hotplug scripts: now that we're using the VM's uuid
it's easier to find the xenopsd state on disk -- there's no need
for a separate 'Interface' link table.

Fixes #25

Signed-off-by: David Scott <dave.scott@eu.citrix.com>