libxl: make devid a type so it is initialized properly

libxl: make devid a type so it is initialized properly

Previously device ids in libxl were treated as integers meaning they
were being initialized to 0, which is a valid device id. This patch
makes devid its own type in libxl and initializes it to -1, an invalid
value.

This fixes a bug where if you try to do a xl DEV-attach multiple
time it will continuously try to reattach device 0 instead of
generating a new device id.

Signed-off-by: Matthew Fioravante <matthew.fioravante@jhuapl.edu>
Reviewed-by: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>