libxl: Fix assignment of devid value returned from libxl__device_nextid

libxl: Fix assignment of devid value returned from libxl__device_nextid

Commit 5420f265 has some misplaced parenthesis that caused devid
to be assigned 1 or 0 based on checking return value of
libxl__device_nextid < 0, e.g.

  devid = libxl__device_nextid(...) < 0

This works when only one instance of a given device type exists, but
subsequent devices of the same type will also have a devid = 1 if
libxl__device_nextid succeeds.  Fix by checking the value assigned to
devid, e.g.

  (devid = libxl__device_nextid(...)) < 0

Signed-off-by: Jim Fehlig <jfehlig@suse.com>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>