qemu: Ignore graphics cookie if port == 0

qemu: Ignore graphics cookie if port == 0

Old libvirt represents

    <graphics type='spice'>
      <listen type='none'/>
    </graphics>

as

    <graphics type='spice' autoport='no'/>

In this mode, QEMU doesn't listen for SPICE connection anywhere and
clients have to use virDomainOpenGraphics* APIs to attach to the domain.
That is, the client has to run on the same host where the domains runs
and it's impossible to tell the client to reconnect to the destination
QEMU during migration (unless there is some kind of proxy on the host).

While current libvirt correctly ignores such graphics devices when
creating graphics migration cookie, old libvirt just sends

    <graphics type='spice' port='0' listen='0.0.0.0' tlsPort='-1'/>

in the cookie. After seeing this cookie, we happily would call
client_migrate_info QMP command and wait for SPICE_MIGRATE_COMPLETED
event, which is quite pointless since the doesn't know where to connecti
anyway. We should just ignore such cookies.

https://bugzilla.redhat.com/show_bug.cgi?id=1376083

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>