qemu: caps: Add lockout for -blockdev if QEMU_CAPS_SCSI_DISK_DEVICE_ID is not present

Avoid regressions by disallowing the BLOCKDEV capability.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>

src/qemu/qemu_capabilities.c

@@ -4331,6 +4331,11 @@ virQEMUCapsInitQMPMonitor(virQEMUCapsPtr qemuCaps,
             virQEMUCapsClear(qemuCaps, QEMU_CAPS_SEV_GUEST);
     }
 
+    /* To avoid guest ABI regression, blockdev shall be enabled only when
+     * we are able to pass the custom 'device_id' for SCSI disks and cdroms. */
+    if (!virQEMUCapsGet(qemuCaps, QEMU_CAPS_SCSI_DISK_DEVICE_ID))
+        virQEMUCapsClear(qemuCaps, QEMU_CAPS_BLOCKDEV);
+
     ret = 0;
  cleanup:
     return ret;