qemu: migration: Disallow migration of read only disk

Currently Libvirt allows attempts to migrate read only disks. Qemu
cannot handle this as read only disks cannot be written to on the
destination system. The end result is a cryptic error message and a
failed migration.

This patch causes migration to fail earlier and provides a meaningful
error message stating that migrating read only disks is not supported.
Signed-off-by: NCorey S. McQuay <csmcquay@linux.vnet.ibm.com>
Reviewed-by: NJason J. Herne <jjherne@linux.vnet.ibm.com>
Reviewed-by: NBoris Fiuczynski <fiuczy@linux.vnet.ibm.com>

src/qemu/qemu_migration.c

@@ -1765,6 +1765,12 @@ qemuMigrationStartNBDServer(virQEMUDriverPtr driver,
         if (!qemuMigrateDisk(disk, nmigrate_disks, migrate_disks))
             continue;
 
+        if (disk->src->readonly) {
+            virReportError(VIR_ERR_OPERATION_UNSUPPORTED,
+                           _("Cannot migrate read-only disk %s"), disk->dst);
+            goto cleanup;
+        }
+
         VIR_FREE(diskAlias);
         if (!(diskAlias = qemuAliasFromDisk(disk)))
             goto cleanup;