Description of problem:
When boot qemu-kvm with "-vnc :-5900", the listening port will not be 0 port of host. The listening port will be changed by host. So qemu-kvm should give some friendly messages.  

Version-Release number of selected component (if applicable):
qemu-kvm-rhev-1.5.3-9.el7.x86_64
3.10.0-34.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1.Boot qemu-kvm with "-vnc :-5900".
# /usr/libexec/qemu-kvm -vnc :-5900 -monitor stdio
2.Check the listening port.
(qemu) info vnc


Actual results:
(qemu) info vnc 
Server:
     address: 0.0.0.0:47569
        auth: none
Client: none

Every time this port is different.

Expected results:
When boot qemu-kvm with "-vnc :-5900", qemu-kvm can not give some warning message or forbids using this port.

Additional info:
I also test "-spice port=0", it will give a error message.
# /usr/libexec/qemu-kvm -vnc :-5900 -spice port=0 -monitor stdio
qemu-kvm: neither port nor tls-port specified for spice