Fix VNC SASL authentication when using a QXL device

ui/vnc.c:vnc_display_open() and spice-server/server/reds.c:do_spice_init()
are both calling sasl_server_init(). If spice_server_set_sasl_appname()
hasn't been called, spice-server will call it with "spice" as an appname,
causing cyrus-sasl to try to use a /etc/sasl2/spice.conf config file rather
than the /etc/sasl2/qemu.conf file that QEMU uses.

When using -spice sasl on the command line, QEMU properly calls
spice_server_set_sasl_appname() to set the SASL appname as "qemu",
but when using a QXL device without using SPICE, spice_server_init()
is called from qemu_spice_add_interface() without setting the appname
to "qemu", which then causes the VNC code to try to use spice.conf
instead of qemu.conf.

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