tools: fix compile errors with -Og

At least gcc-4.8 and older fails to recognize that err is always
initialized, the build fails:
  xc_cpupool.c: In function 'xc_cpupool_removecpu':
  xc_cpupool.c:168:5: error: 'err' may be used uninitialized in this function [-Werror=maybe-uninitialized]
    return err;

Fix this in blktap2 and libxc.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>

tools/blktap2/drivers/tapdisk-vbd.c

@@ -516,7 +516,7 @@ tapdisk_vbd_free_stack(td_vbd_t *vbd)
 int
 tapdisk_vbd_open_stack(td_vbd_t *vbd, uint16_t storage, td_flag_t flags)
 {
-	int i, err;
+	int i, err = 0;
 
 	vbd->flags   = flags;
 	vbd->storage = storage;
@@ -932,7 +932,7 @@ tapdisk_vbd_open_image(td_vbd_t *vbd, td_image_t *image)
 static int
 tapdisk_vbd_close_and_reopen_image(td_vbd_t *vbd, td_image_t *image)
 {
-	int i, err;
+	int i, err = 0;
 
 	td_close(image);
 
@@ -972,7 +972,7 @@ tapdisk_vbd_pause(td_vbd_t *vbd)
 int
 tapdisk_vbd_resume(td_vbd_t *vbd, const char *path, uint16_t drivertype)
 {
-	int i, err;
+	int i, err = 0;
 
 	if (!td_flag_test(vbd->state, TD_VBD_PAUSED)) {
 		EPRINTF("resume request for unpaused vbd %s\n", vbd->name);

tools/libxc/xc_cpupool.c

@@ -151,7 +151,7 @@ int xc_cpupool_removecpu(xc_interface *xch,
                          int cpu)
 {
     unsigned retries;
-    int err;
+    int err = 0;
     DECLARE_SYSCTL;
 
     sysctl.cmd = XEN_SYSCTL_cpupool_op;