qemu: Let empty default VNC password work as documented

CVE-2016-5008

Setting an empty graphics password is documented as a way to disable
VNC/SPICE access, but QEMU does not always behaves like that. VNC would
happily accept the empty password. Let's enforce the behavior by setting
password expiration to "now".

https://bugzilla.redhat.com/show_bug.cgi?id=1180092



Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
(cherry picked from commit bb848fee)
(cherry picked from commit d933f68e)