qemuimagetest: fix erroneous ps errors when qemu couldn't be started
The helper script was printing an error to stdout when it couldn't find
any qemu child processes; output this error to stderr instead and
redirect stderr to /dev/null when running from qemuimage-testlib so that
QEMUPID is actually blank if there are no qemu instances found.
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
