spice: Disallow use of gl + TCP port

Currently, virgl support has to go through a local unix socket, trying
to connect to a VM using -spice gl through spice://localhost:5900 will
only result in a black screen.
This commit errors out when the user tries to start a VM with both GL
support and a port/tls-port set.
This would fit better in spice-server, but currently QEMU does not call
into spice-server when parsing 'gl' on its command line, so we have to
do this check in QEMU instead.

Signed-off-by: Christophe Fergeau <cfergeau@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 1457955672-28758-1-git-send-email-cfergeau@redhat.com

[ applied codestyle fix: break long line ]

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
