qemu-iotests: add option in common.qemu for mismatch only

Add option to echo response to QMP / HMP command only on mismatch.

Useful for ignore all normal responses, but catching things like
segfaults.

Signed-off-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

tests/qemu-iotests/common.qemu

@@ -50,6 +50,8 @@ _in_fd=4
 #
 # If $silent is set to anything but an empty string, then
 # response is not echoed out.
+# If $mismatch_only is set, only non-matching responses will
+# be echoed.
 function _timed_wait_for()
 {
     local h=${1}
@@ -58,14 +60,18 @@ function _timed_wait_for()
     QEMU_STATUS[$h]=0
     while IFS= read -t ${QEMU_COMM_TIMEOUT} resp <&${QEMU_OUT[$h]}
     do
-        if [ -z "${silent}" ]; then
+        if [ -z "${silent}" ] && [ -z "${mismatch_only}" ]; then
             echo "${resp}" | _filter_testdir | _filter_qemu \
                            | _filter_qemu_io | _filter_qmp | _filter_hmp
         fi
         grep -q "${*}" < <(echo "${resp}")
         if [ $? -eq 0 ]; then
             return
+        elif [ -z "${silent}" ] && [ -n "${mismatch_only}" ]; then
+            echo "${resp}" | _filter_testdir | _filter_qemu \
+                           | _filter_qemu_io | _filter_qmp | _filter_hmp
         fi
+
     done
     QEMU_STATUS[$h]=-1
     if [ -z "${qemu_error_no_exit}" ]; then