Description of problem:
there is a bug 870703 in rhel 6.4, and also hit it in rhel7, but there are some difference. QMP monitor will hang or fail to prompt the error information at the first time after run wrong command.

Version-Release number of selected component (if applicable):
host info:
# uname -r && rpm -q qemu-kvm
3.7.0-0.34.el7.x86_64
qemu-kvm-1.3.0-5.el7.x86_64
guest info:
qemu-guest-agent-1.3.0-5.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1.boot guest with virtio-serial.
e.g:...-chardev socket,path=/tmp/qga.sock,server,nowait,id=qga0 -device virtio-serial -device virtserialport,chardev=qga0,name=org.qemu.guest_agent.0,id=sr0
2.start qemu-ga server in guest.
3.connect the chardev socket in host side for sending commands. 
# nc -U /tmp/qga.sock

Actual results:
- scenario 1,
# nc -U /tmp/qga.sock
{"execute":"guest-ping"}
{"return": {}}
{"execute":"guest-ping"    <-----miss '}'
                           <-----monitor hang and cann't run any commands 
{"execute":"guest-ping"}
                           
Restart the qemu-ga server in guest, then it return to works fine.
# /usr/bin/qemu-ga restart
{"execute":"guest-ping"}
{"return": {}}

# nc -U /tmp/qga.sock
{"execute":"guest-ping"}{    <-----more '{'
{"return": {}}
{"execute":"guest-ping"}{  
                             <-----monitor hang but can run commands again
{"execute":"guest-ping"}
{"return": {}}

- scenario 2, it fail to prompt the error information at the first time.
# nc -U /tmp/qga.sock
{"execute":"guest-ping"}
{"return": {}}
{"execute":"guest-ping"}{"e   <-----more '{"e'
{"return": {}}
{"execute":"guest-ping"}{"e
{"error": {"class": "GenericError", "desc": "Invalid JSON syntax"}}
{"error": {"class": "GenericError", "desc": "JSON parse error, invalid keyword `ing'"}}
{"error": {"class": "GenericError", "desc": "Invalid JSON syntax"}}
{"error": {"class": "GenericError", "desc": "JSON parse error, invalid keyword `e'"}}
{"execute":"guest-ping"}   
{"return": {}}

# nc -U /tmp/qga.sock
{"execute":"guest-ping"}
{"return": {}}
{"execute":"guest-ping"}{"e   <-----more '{"e'
{"return": {}}
{"execute":"guest-ping"}{"e
{"error": {"class": "GenericError", "desc": "Invalid JSON syntax"}}
{"error": {"class": "GenericError", "desc": "JSON parse error, invalid keyword `ing'"}}
{"error": {"class": "GenericError", "desc": "Invalid JSON syntax"}}
{"error": {"class": "GenericError", "desc": "JSON parse error, invalid keyword `e'"}}
{"execute":"guest-ping"}{"e
{"return": {}}
{"execute":"guest-ping"}{"e
{"error": {"class": "GenericError", "desc": "Invalid JSON syntax"}}
{"error": {"class": "GenericError", "desc": "JSON parse error, invalid keyword `ing'"}}
{"error": {"class": "GenericError", "desc": "Invalid JSON syntax"}}
{"error": {"class": "GenericError", "desc": "JSON parse error, invalid keyword `e'"}}
{"execute":"guest-ping"}{"e
{"return": {}}
{"execute":"guest-ping"}{"e
{"error": {"class": "GenericError", "desc": "Invalid JSON syntax"}}
{"error": {"class": "GenericError", "desc": "JSON parse error, invalid keyword `ing'"}}
{"error": {"class": "GenericError", "desc": "Invalid JSON syntax"}}
{"error": {"class": "GenericError", "desc": "JSON parse error, invalid keyword `e'"}}
...

Expected results:


Additional info: