qxl: fix qxl_conn_get_modes

Call qxl_add_monitors_config_modes() unconditionally.  Do all sanity
checks in that function.

Fix sanity checks.  monitors_config is the current monitor
configuration, whereas client_monitors_config is the configuration
requested by the spice client.  So when filling the mode list, based on
the spice client request, we need to look at
client_monitors_config->count not monitors_config->count.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1488363154-6889-5-git-send-email-kraxel@redhat.com