qemu: set QEMU_CAPS_DEVICE_VIDEO_PRIMARY cap flag in QMP detection

When qemu >= 1.20, it is safe to use -device for primary video
device as described in 4c993d8a.
So, we are missing the cap flag in QMP capabilities detection, this
flag can be initialized safely in virQEMUCapsInitQMPBasic.

src/qemu/qemu_capabilities.c

@@ -2418,6 +2418,7 @@ virQEMUCapsInitQMPBasic(virQEMUCapsPtr qemuCaps)
     virQEMUCapsSet(qemuCaps, QEMU_CAPS_MACHINE_OPT);
     virQEMUCapsSet(qemuCaps, QEMU_CAPS_DUMP_GUEST_CORE);
     virQEMUCapsSet(qemuCaps, QEMU_CAPS_VNC_SHARE_POLICY);
+    virQEMUCapsSet(qemuCaps, QEMU_CAPS_DEVICE_VIDEO_PRIMARY);
 }
 
 /* Capabilities that are architecture depending