Turn off tracing around pid/chroot check

In the error case, we get a spew of output as this check goes though
every pid checking if its in the chroot.  Disable tracing around the
call.

Change-Id: Ie84f12974755c0c2c51d7e7697337ed9b32a4a1c

lib/common-functions

@@ -135,6 +135,10 @@ function eval_run_d () {
 }
 
 function kill_chroot_processes () {
+    local xtrace
+    xtrace=$(set +o | grep xtrace)
+    set +o xtrace
+
     if [ -z "${1}" ]; then
         echo "ERROR: no chroot directory specified"
         exit 1
@@ -149,6 +153,8 @@ function kill_chroot_processes () {
             sudo kill $pid
         fi
     done
+
+    $xtrace
 }
 
 function cleanup_build_dir () {