Description of problem:
fail to use "-serial none" to disable all serial ports, it's confusing for users for the qemu-kvm doc explain, doc should be improved if the "-serial none" is not the global option thing. 
Please help move it to the correct component if you decide it, thanks.

Version-Release number of selected component (if applicable):
host info:
# uname -r && rpm -q qemu-kvm-rhev
3.10.0-121.el7.x86_64
qemu-kvm-rhev-1.5.3-60.el7ev.x86_64
guest info:
rhel7: kernel-3.10.0-121.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1.check the qmeu-kvm doc for '-serial none' explains.
...
       -serial dev
           Redirect the virtual serial port to host character device dev. The default device is "vc" in graphical mode
           and "stdio" in non graphical mode.

           This option can be used several times to simulate up to 4 serial ports.

           Use "-serial none" to disable all serial ports.
...
2.boot up a KVM guest with "-serial none -serial unix:/tmp/qemu-serial0,server,nowait".
3.transfer some characters via serial port.
(host) # nc -U /tmp/qemu-serial${PORT_NUMBER}
(guest)# cat /dev/urandom > /dev/ttyS${PORT_NUMBER}

Actual results:
it can transfer characters via serial port0, fail to use "-serial none" to disable all serial ports.
guest ]# echo LuoSibiao > /dev/ttyS1
 host ]# nc -U /tmp/qemu-serial0
LuoSibiao

Expected results:
it's confusing for users for the qemu-kvm doc explain, doc should be improved if the "-serial none" is not the global option thing. 

Additional info: