I have just sent a patch to upstream. There will be used the readline functionality for qmp on stdio as it is used for hmp on stdio. After it will be approved and committed to upstream, I'll send patch for rhel.

For now into RHEL will be backported the echo functionality from upstream. If the readline mode will be accepted in upstream then we could backport it too.

---Reproduced on qemu-kvm-0.12.1.2-2.295.el6.x86_64.
Steps:
1. start a QMP monitor on stdio by:
# /usr/libexec/qemu-kvm -qmp stdio
2. send {"execute":"qmp_capabilities"}

Actual Result:
# /usr/libexec/qemu-kvm -qmp stdio
VNC server running on `::1:5900'
{"QMP": {"version": {"qemu": {"micro": 1, "minor": 12, "major": 0}, "package": "(qemu-kvm-0.12.1.2)"}, "capabilities": []}}
{"return": {}}  <--- without echoing {"execute":"qmp_capabilities"}

---Verified on qemu-kvm-0.12.1.2-2.340.el6.x86_64 with same steps described above.
Actual Result:
# /usr/libexec/qemu-kvm -qmp stdio
VNC server running on `::1:5900'
{"QMP": {"version": {"qemu": {"micro": 1, "minor": 12, "major": 0}, "package": "(qemu-kvm-0.12.1.2)"}, "capabilities": []}}
{"execute":"qmp_capabilities"} <---echoing {"execute":"qmp_capabilities"}
{"return": {}}

---Conclusion:
As per above, this issue has been fixed correctly.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

http://rhn.redhat.com/errata/RHBA-2013-0527.html