xenlight: sort the VIFs into VIF.device order before we attach them to a new VM

Linux frontends enumerate VIFs in the order they're found in 'xenstore-ls'.
The order of 'xenstore-ls' is determined by the order of 'xenstore-write'.
The order of 'xenstore-write' is determined by the order of 'nics' passed
to the libxl domain_create call.

Fixes #31

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