qxl: Fix new function name for spice-server library

The new spice-server function to limit the number of monitors (0.12.6)
changed while development from spice_qxl_set_monitors_config_limit to
spice_qxl_max_monitors (accepted upstream).
By mistake I post patch with former name.
This patch fix the function name.

Signed-off-by: Frediano Ziglio <fziglio@redhat.com>
Acked-by: Christophe Fergeau <cfergeau@redhat.com>
Acked-by: Martin Kletzander <mkletzan@redhat.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
