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>

tools/libxl/libxl.c

@@ -1781,7 +1781,7 @@ void libxl__device_vtpm_add(libxl__egc *egc, uint32_t domid,
     back = flexarray_make(gc, 16, 1);
 
     if (vtpm->devid == -1) {
-        if ((vtpm->devid = libxl__device_nextid(gc, domid, "vtpm") < 0)) {
+        if ((vtpm->devid = libxl__device_nextid(gc, domid, "vtpm")) < 0) {
             rc = ERROR_FAIL;
             goto out;
         }
@@ -2877,7 +2877,7 @@ void libxl__device_nic_add(libxl__egc *egc, uint32_t domid,
     back = flexarray_make(gc, 18, 1);
 
     if (nic->devid == -1) {
-        if ((nic->devid = libxl__device_nextid(gc, domid, "vif") < 0)) {
+        if ((nic->devid = libxl__device_nextid(gc, domid, "vif")) < 0) {
             rc = ERROR_FAIL;
             goto out;
         }
@@ -3258,7 +3258,7 @@ int libxl__device_vkb_add(libxl__gc *gc, uint32_t domid,
     back = flexarray_make(gc, 16, 1);
 
     if (vkb->devid == -1) {
-        if ((vkb->devid = libxl__device_nextid(gc, domid, "vkb") < 0)) {
+        if ((vkb->devid = libxl__device_nextid(gc, domid, "vkb")) < 0) {
             rc = ERROR_FAIL;
             goto out;
         }
@@ -3359,7 +3359,7 @@ int libxl__device_vfb_add(libxl__gc *gc, uint32_t domid, libxl_device_vfb *vfb)
     back = flexarray_make(gc, 16, 1);
 
     if (vfb->devid == -1) {
-        if ((vfb->devid = libxl__device_nextid(gc, domid, "vfb") < 0)) {
+        if ((vfb->devid = libxl__device_nextid(gc, domid, "vfb")) < 0) {
             rc = ERROR_FAIL;
             goto out;
         }