protocol: Define an enumeration for Quality Indicator messages

These messages are used to transmit streaming quality indicators from
SPICE client to SPICE server.

The message data structure is defined in spice-common/spice.proto:

    message {
        Data data @to_ptr;
    } @declare quality_indicator;

Signed-off-by: Kevin Pouget <kpouget@redhat.com>
Acked-by: Frediano Ziglio <fziglio@redhat.com>

spice/enums.h

@@ -468,6 +468,7 @@ enum {
     SPICE_MSGC_MAIN_MIGRATE_END,
     SPICE_MSGC_MAIN_MIGRATE_DST_DO_SEAMLESS,
     SPICE_MSGC_MAIN_MIGRATE_CONNECTED_SEAMLESS,
+    SPICE_MSGC_MAIN_QUALITY_INDICATOR,
 
     SPICE_MSGC_END_MAIN
 };