qemu: avoid throwing exceptions in the cleanup path which could cause infinite loops

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
qemu-vif-script: correct minor errors
VIF: add the concept of an 'Interface' which can be used to look up the 'VIF'

xenopsd knows when it is creating a tap, or a vif device and can
link these to the correspoding API-visible VIF. The python library
can then look up the VIF metadata given only the interface name
(eg tap1.2)

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