qemu: set seamless migration capability

As we switched to setting capabilities based on QMP communication,
qemu seamless-migration capability was not set. In the -help output
this knob is called seamless-migration=[on|off]. The equivalent in
QMP world is SPICE_MIGRATE_COMPLETED event (qemu upstream commit
2fdd16e2).

src/qemu/qemu_capabilities.c

@@ -1908,6 +1908,8 @@ qemuCapsProbeQMPEvents(qemuCapsPtr caps,
 
         if (STREQ(name, "BALLOON_CHANGE"))
             qemuCapsSet(caps, QEMU_CAPS_BALLOON_EVENT);
+        if (STREQ(name, "SPICE_MIGRATE_COMPLETED"))
+            qemuCapsSet(caps, QEMU_CAPS_SEAMLESS_MIGRATION);
         VIR_FREE(name);
     }
     VIR_FREE(events);