libxl: Set vfb and vkb devid if not done so by the caller

libxl: Set vfb and vkb devid if not done so by the caller

Other devices set a sensible devid if the caller has not done so.
Do the same for vfb and vkb.  While at it, factor out the common code
used to determine a sensible devid, so it can be used by other
libxl__device_*_add functions.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>