qemuimagetest: fix erroneous ps errors when qemu couldn't be started

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.

(From OE-Core rev: 7c2137a07cca8e1d40d3c8b4b2c6321d80f2b1de)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>