libxl: drop domid field from libxl_device_*

libxl: drop domid field from libxl_device_*

All functions which add a device to a domain already take a domid
argument and the callers typically write the same value to the
structure right before making the call.

Functions which delete a device typically do not but adding this field
makes the interface more consistent anyway and all callers have the
domid to hand.

All functions which return a libxl device structure are given a domid
as a paramter and the caller therefore already knows which domain it
is dealing with.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>