qemu: vnc: error out for invalid port number
In the XML we have the vnc port number, but QEMU takes on command line
a vnc screen number, it's port-5900. We should fail with error message
that only ports in range [5900,65535] are valid.
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1164966
Signed-off-by: Pavel Hrdina <phrdina@redhat.com>